Military Flight Safety

ramana · Post by **ramana** » 10 May 2023 21:39

Kaypius describes the collective control system and its importance for ALH with its hingeless rotor

Link: https://kaypius.com/2022/10/23/thoughts ... t-21-2022/

Understanding the “flying brick”

The Print article raises this important issue of “collective failure“. The collective lever in a helicopter is in essence the power lever. Moving the lever up or down controls the pitch of all rotor blades — collectively and by the same amount. Losing collective (or cyclic control) is worse than losing one or both engines. Let me explain why.

A helicopter is essentially a flying brick, kept in the air by spinning rotor blades that need cyclic and collective pitch changes for each and every condition of flight. The helicopter pilot’s last line of defence in an extreme situation of losing both engines (autorotation) is still way better than losing control over the rotor disc. A “collective failure” of main rotor essentially does that. It decouples the power lever (collective) from the main rotors, leaving the pilot to execute a “deadstick”, “cyclic-only” approach ending with a high-speed run-on landing.

“Collective failure” in a rigid rotor system

Unlike aeroplanes, helicopter landing gear (skid or wheel) is rather averse to high-speed run-on landings — about 40-60 knots (80-120 kmph) would be the outer edge of certified envelope on a firm, level, even surface; not to talk of water-landing where a running landing may “cartwheel” the machine over. Think of “control failure” as driving on a winding mountain road and suddenly losing the steering wheel. Wouldn’t properly certified automobiles absolutely preclude such an occurrence? Now think helicopters flying over treacherous, hostile terrain.

The ALH has a hingeless rotor system designed for high agility and performance stretching from sea level to super high altitude. It’s an elite club where only a few OEMs can safely compete. The one that keeps it all together in flight is the pilot, acting through the propulsion system (comprising engines and the rotor system) and the flight control chain. As opposed to helicopters with fully-articulated rotor systems that “feather”, “flap” and “drag” about mechanical hinges, rigid-rotor helicopters like ALH achieve all these three movements about elastomeric bearings and “virtual hinges”. The blades are forced to “twist and shout” through daunting aerodynamic odds by servo jacks holding the blades against flex-beam, elastomeric, not mechanical bearings. So, should any upper control linkages or primary servo jacks fail, the blades will almost instantaneously “offload” and spring back to ‘installed angle’ or one that offers least resistance to relative airflow.

What did we learn from 2019?

In short, should a “collective failure” occur, on the ALH, the pilot will be left with a high rate of descent, one-way-ticket-down, with an impending high-speed, cyclic-only landing. What if the terrain below is “hostile”?

It is for the reader to surmise the almost impossible odds that pilots flying Lt Gen Ranbir Singh & Co in the ALH Mk 3 on Oct 24, 2019 faced when they experienced “collective failure” in the most inauspicious location — hilly terrain with no clear area to carry out a running landing. In the event, the pilots skillfully managed to put down the helicopter in a valley within the maxim of “if you cannot save the airframe, save the pax”. Such feats are not repeatable or guaranteed for success each time; nor are such “failures” or “recovery procedures” standardised or described in OEM manuals — they cannot be; they are NOT supposed to fail.

Oct 2019 crash:

Link: https://kaypius.com/2019/10/25/army-alh ... y-lessons/
As they are early thoughts, not much analysis.

But here is the snippet from Print:

The accident brings to mind the deadly crash in 2019 of an ALH Dhruv, in which Lt Gen Ranbir Singh, who was the Northern Army commander at that time, had a miraculous escape after the chopper carrying him and seven others crash-landed in Jammu and Kashmir’s Poonch region.

While the findings of the court of inquiry into that crash is not public, sources in the defence establishment said that the crash happened after the “collective”, which controls the power to the rotors and back, broke.

This, the sources said, was a manufacturing defect.

The crash might have broken the rods. The rod joints at the bell crank could have come loose leading to loss of control.
Maybe even better is a cotter pin so the bolt doesn't separate if the washer is loose.

Anujan · Post by **Anujan** » 10 May 2023 22:10

This is a twitter thread by Indranil that goes into design choices of the control rod in ALH

https://threadreaderapp.com/thread/1643 ... 39874.html

chetak · Post by **chetak** » 10 May 2023 22:49

Anujan wrote:This is a twitter thread by Indranil that goes into design choices of the control rod in ALH

https://threadreaderapp.com/thread/1643 ... 39874.html

This is the BK 117. The pedigree of the ALH springs from the origins of this MBB line

Had the good fortune to fly in this beauty in 1986.

Its configuration could be changed to suit a new mission profile in barely an hour or two, at the most

basant · Post by **basant** » 11 May 2023 07:21

ramana wrote:The serrated washers must be to connect the control rods to the bell crank. Thats the only place the CCR would need such a fastener joint.
So what fatigue this CCR is seeing? If the serrated washer is installed wrong, the nut comes loose and you lose CCR authority. There are two bell cranks. Despite being engineers the CEMILAC use imprecise language. Sad.

Ramana garu, a reply from Kiran Raghuram touches the issue:

E.R. Sakthivel DFI @sakthivel_cit93
Since maintenance personnel cant get it right , when servicing control rods,
They want a simpler redesign fr easy maintenance
It doesn't mean that old rod designs are faulty
Bcos they were also cleared by the same CEMILAC

shiv_cybersurg @shiv_cybersurg
Not design fault. Material fatigue. Aluminium rods breaking.

Kiran Raghuram @kiranraghu_7
No sir, there are rods of the same design which have completed almost 2000 hrs of continuous service. Improperly assembled serrated washers cause premature failures. This was proven both theoretically and practically, independently by three agencies.

ramana · Post by **ramana** » 11 May 2023 10:21

Ok let me ask on twitter.

ramana · Post by **ramana** » 12 May 2023 00:37

Rahul Singh writes in HT on Twitter

https://twitter.com/rahulsinghx/status/ ... 69600?s=20

and

ramana · Post by **ramana** » 12 May 2023 01:17

Key parts of the above report:
The expert committee released their letter on 23 April 2023 to HAl and three services.

Analysis was conducted by CSIR's NAL, HAL's RWR&DC.
Postulated most probable cause is a technical failure. (This tells me they are using fault tree analysis)

Both confirmed that control rods see a drastic reduction in fatigue life when there is an error in the assembly of serrated washers in the booster control rods.
This was confirmed experimentally by the HAL center.
They recommended short-term and long-term fixes foir ALH and LCH which shares parts.
The proposed long-term fix is a material change to steel and tolerant to assembly errors.
This shall be implemented in six months to one year for the design, development, and qualification of these control rods.
Parallelly they give clearance for 100 hours of flight after some checks and for 500 hours or one-year whichever comes first after HAL completes two tests.
1) Flight test of two fully instrumented helicopters ALH and LCH to identify the multi-axis load on the control rods.
2) fatigue testing of the rods with correctly assembled serrated washers to confirm original capability.
(Comment this should have been done by HAL regardless once ALH was flight certified in year one. The designer has to know his design margin always.)

CEMILAC also throws in the idea of Accelerated Life Testing(ALT) to find other components' failure modes.

This in my view is a witch hunt and scare yourself project. ALT works for simple components with known load paths. In aerospace one relies on design margin to handle the unknown unknowns.
The fact that ALH flew a cumulative flight regime of 300,000 hours with correctly installed components shows ALT is a moot exercise.
Nomenclature issue.
I don't see why they call it a design review when what is recommended is a result of a Failure/Flight Investigation Team(FIT) report. What is recommended is a corrective action.
HAL has to issue a report after the new control rods are qualified and the two tests are completed.
It's for the services to monitor that their units have the design changes implemented.

PS: I don't understand how the loss of pre-load at a bolted connection is affecting fatigue life in control rod linkage which doesn't see tensile loads. And some of the crashes are after 2 hours after recertifying.
Need to see the assembly drawing.

Overall good RCA and Corrective Action.

pravula · Post by **pravula** » 12 May 2023 01:45

So works as designed, but scope to improve said design. That seems like a solid iterative approach. Kudos…esp being able to replicate experimentally.

basant · Post by **basant** » 12 May 2023 07:26

Highly Accelerated Life Testing (HALT) and Highly Accelerated Stress Screening (HASS) are necessary processes in design and development. It is always beneficial to watch out for early warnings that may otherwise be not visible. They may be necessary to certify the improved designs too.

HALT vs HASS – what is the difference?
The terms HALT (Highly Accelerated Life Tests) and HASS (Highly Accelerated Stress Screen) are both used for accelerating testing. There is differentiation in the terms based on the stage of development of the product. HALT testing is done earlier in development on bare components and sub-assemblies of a product, which is a functional sub-assembly. There is an emphasis on rapid temperature changes as accelerate stress. Completed products ready for manufacturing, on the other hand, are subject to HASS testing, with greater focus on mechanical testing and more stable extreme temperatures, as the components within a product are less subject to rapid temperature changes.

I remember seeing the test facility in one of Anantha Krishnan's videos on LCA. Similar facilities exist in CABS (under Environmental Test Facility) and even for electronic systems such as radars.

Nalla Baalu · Post by **Nalla Baalu** » 13 May 2023 02:59

Very valid comments.

HALT is a design derisking tool to surface failure modes of a component design that is not 'field-proven' and failure modes are poorly understood. While HASS is necessarily production derisking tool that too if there is a known 'infant-mortality' failure mode.

As a systems integrator, in absence of field proven components, you either spec Reliability targets to supplier and verify/validate supplier responses. Or set up test facilities to do the needful in-house. The latter is a lofty aim but as always time/budget/infra/talent constraints abound given variety of components a single integrated system involves.

basant wrote:Highly Accelerated Life Testing (HALT) and Highly Accelerated Stress Screening (HASS) are necessary processes in design and development. It is always beneficial to watch out for early warnings that may otherwise be not visible. They may be necessary to certify the improved designs too.

ramana · Post by **ramana** » 13 May 2023 05:09

ALH per HAL has 3 lakh hour cumulative service and about 300 helicopters built.
So the ALT is not useful for ALH.
Things that could go wrong are by now patent.

basant · Post by **basant** » 13 May 2023 11:29

ramana wrote:ALH per HAL has 3 lakh hour cumulative service and about 300 helicopters built.
So the ALT is not useful for ALH.
Things that could go wrong are by now patent.

Ramana garu, let's look at it this way. There have been about 20 accidents since 2002 over 3L+ hours. That compares well wrt to Chinook (10+) and Blackhawk (7+) per 100000 hrs. However, considering only technical reasons for its problem such as component failure (about 5-6/20), almost all of them are related to control rods. So if even that is eliminated, it could have near zero crash rate and could come close to Tejas! Even if we can save 1 fatal accident, the cost of the helo alone would cover the investment (and we cannot put a number against lives).

Equally important is the public perception. The officers, and some well meaning too, would not intentionally/unintentionally smear the safety perception. Because, even today some of them do not appreciate its safety record, instead keep deriding it at the drop of a hat.

300000 hours is over hundreds of helos. But as they age, the fatigue aspect becomes a factor. In case the test establishes more than designed lifetime, it is even better. We can fly the helos longer!

chetak · Post by **chetak** » 13 May 2023 12:23

basant wrote:
ramana wrote:ALH per HAL has 3 lakh hour cumulative service and about 300 helicopters built.
So the ALT is not useful for ALH.
Things that could go wrong are by now patent.
Ramana garu, let's look at it this way. There have been about 20 accidents since 2002 over 3L+ hours. That compares well wrt to Chinook (10+) and Blackhawk (7+). However, considering only technical reasons for its problem such as component failure (about 5-6/20), almost all of them are related to control rods. So if even that is eliminated, it could have near zero crash rate and could come close to Tejas! Even if we can save 1 fatal accident, the cost of the helo alone would cover the investment (and we cannot put a number against lives).

Equally important is the public perception. The officers, and some well meaning too, would not intentionally/unintentionally smear the safety perception. Because, even today some of them do not appreciate its safety record, instead keep deriding it at the drop of a hat.

300000 hours is over hundreds of helos. But as they age, the fatigue aspect becomes a factor. In case the test establishes more than designed lifetime, it is even better. We can fly the helos longer!

saar,

let this statistical analysis go, it is leading nowhere

These accidents are bunched up in a very small time frame, and that too, affecting multiple operators and, at times, multiple fatalities, injuries, and also, the loss of hull

It was considered serious enough to cause the grounding of the ALH twice within a very short time frame, these two major events which are unprecedented in our aviation MIL service history

the causes are endemic, systemic, and the alignment mechanics of the swiss cheese model is apparent and the remedial actions will emerge from a composite organizational introspection, done dispassionately, and without institutional bias.

The basic flaw is design. Every aspect of manufacturing (from manufacturing shop floor processes to detailed inspection requirements, it is the design dept that specifies what is to be checked, tolerances allowed, surface finishes and treatments required or monitored and how the specific part is to be declared acceptable as it passes the accept or reject criteria) and maintenance processes (including details of schedules, the required processes, the mandated time frame, lab tests, recalibration, and visual criteria required) is design mandated or design specified. There is no getting away from this one single and vital fact

In any accident investigation, the early involvement of the design dept will help in fine tuning the main thrust of the investigation. Far too many investigations are closed by some illiterate bumpkin heading up the CoI who says with ego fuelled authority, and unwarranted bravado that "isolated instance" of material failure was the reason

such jokers are not the descendants of the "Wright brothers" family but are the descendants from the wrong side of the aviation family, whose patriarchs founded the "Wrong brothers" family lineage

The investigation should have gone on to check out:

Incorrect Material Selection.
Inappropriate Heat Treat Condition.
Corrosive Attack.
Buckling Load.
Creep (Stress Ruptures)
Thermal Shock
Functional Failure
and a host of other reasons depending on the criticality of the part involved

Failure mode analysis is not often clearly understood, let alone it being implemented, so no follow up is usually done

A Failure Effect documents the consequence of a failure occurring. Most FMEA methodologies use Failure Effects to capture the safety, environmental, and production impacts associated with a failure.

needlessly depending on external agencies, CABS and its catastrophic idly failure or NAL and its entirely avoidable saras crash, two "accidents" that resulted in the death of all on board, do not really inspire confidence

It certainly did not require another IA ALH to crash, (with a fatality) just to prove this moot point

we lack the FAA institutionalized mechanism of the AD (airworthiness directive), now adopted largely by almost all national aviation authorities.

This is a all hands on deck situation and not the bitter turf wars that seem to have broken out in the background

ramana · Post by **ramana** » 13 May 2023 14:18

Well said. Thanks for the clarity.

wig · Post by **wig** » 14 May 2023 16:45

https://www.msn.com/en-in/news/other/ha ... b5da6&ei=7

HAL To Replace Aluminium Control Rods Of Advanced Light Helicopters With Ones Made Of Steel
extracted

Many of the problems appear to be traced to the control rod of the chopper and the metal that is used to manufacture it. So far, the ALHs the armed forces have (about 140 with the Army, 70 with the Air Force and 20 with the Navy) have had aluminium control rods. But now, Hindustan Aeronautics, the original manufacturer of the helicopter, after discussions with the armed forces officials have decided to replace the aluminium control rods with ones made of steel.

the machines can fly for about 100 hours after which they will be tested

HAL officials said that the replacement programme will begin shortly. A steel control rod, they said, would be easier for maintenance. The replacement will take about one year. The chopper, otherwise, entirely acceptable by the armed forces has been problematic because of the control rod. With the new steel rods in place, the old problems can become a thing of the past.

basant · Post by **basant** » 14 May 2023 23:44

Chetak ji, no saar for me please.

let this statistical analysis go, it is leading nowhere

Accidents to helos that operate many times at the edge of the envelop is to be expected, but one must strive to minimize them as much as possible. Not everything can be, or should be, blamed on design. If tomorrow maintenance crew replace control rods of Dhruv with that of LUH, even if it doesn't fit properly, will it be a design problem? I don't think so.

There is a reason to look at the statistics and without that no meaningful analysis can be done. Number of accidents is very small, and there is no reason to assume that all accidents are to happen at regular intervals. Just as HAL did not boast in 2022 that crash rate is practically 0, we should also look at the data dispassionately. Of the 3 accidents that happened this year, each leading to grounding, the 1st two have been analyzed by independent teams, and the issue of serrated washers was proved experimentally. The last one I believe will take more time as intact airframe is not available. So let us not rush to blame the investigators as buffoons, they know what they are doing. Maintenance errors happen everywhere, in fact 43% of class C or lower accidents happened in the USA on ground were preventable according to their own reports in 2022. Last month, US Army grounded AH-64 after 2 deadly crashes. And these were in production since 1975, close to 50 years!

I do not understand how CABS and NAL are brought into this discussion and not Tejas that is actually produced in numbers performing safely ever since the first the flight.

MeshaVishwas · Post by **MeshaVishwas** » 15 May 2023 19:45

Airbus regrets to inform that today at approx. 12:45 an accident involving a Learjet aircraft of GFD, a subsidiary of Airbus, occurred at Hohn Air Base in the north of Germany. The aircraft impacted the ground on takeoff within the air base perimeter resulting in the tragic death of both pilots on board. Airbus wishes to express its heartfelt condolences to the families affected by this tragic accident. GFD is fully cooperating with the authorities to determine the root causes of the accident. The investigation is led by the Bundesstelle für Flugunfalluntersuchungen (German Federal Bureau of Aircraft Accident Investigation) in Braunschweig. The aircraft was departing Hohn for a training mission with German Air Force Air Traffic Controllers in the north of Germany. GFD and Airbus will provide updates as they become available.

-@AirbusDefence on Twitter

basant · Post by **basant** » 15 May 2023 20:43

Interesting description. This April IAF (Israeli, not Indian) Apache attack helicopter lands at Kibbutz Ga’ash as technical fault forces it out of Independence Day flyby. The IDF stressed that it was not an emergency landing!

chetak · Post by **chetak** » 15 May 2023 22:28

basant wrote:Interesting description. This April IAF (Israeli, not Indian) Apache attack helicopter lands at Kibbutz Ga’ash as technical fault forces it out of Independence Day flyby. The IDF stressed that it was not an emergency landing!

Some landings are precautionary landings and it is made at the sole discretion and professional assessment of the captain of the aircraft.

while there may have been no actual emergency but even a feeling of unease in managing the aircraft, especially in crowded spectator situations like some major public fly past, a wise captain would take this option, rather than "press on regardless" and perhaps land up in a full blown emergency with little or no options on where to land the aircraft safely without endangering anyone...

BTW, it takes big hairy testimonials to do what this captain did (leave the fly past), especially in the full glare of national TV cameras, and in full public view of the crowds watching the fly past, and maybe, with senior govt ministers present in the crowd, to make a precautionary landing at a safe place

ramana · Post by **ramana** » 16 May 2023 07:13

Chetak, I looked up BK-117 manuals and they say the control rods for both the main rotor and aft rotor are push pull rod linkages. The main rotor has a hydraulic booster package to help.
So how is a push-pull system for the aft rotor seeing fatigue damage with a loose bolt?
Have you seen the ALH collective control rod system?

chetak · Post by **chetak** » 16 May 2023 11:47

ramana wrote:Chetak, I looked up BK-117 manuals and they say the control rods for both the main rotor and aft rotor are push pull rod linkages. The main rotor has a hydraulic booster package to help.
So how is a push-pull system for the aft rotor seeing fatigue damage with a loose bolt?
Have you seen the ALH collective control rod system?

Vibrations in a helo are a very complex phenomena because of the main rotor and tail rotor. These are primarily dependent on the speed of rotation of the rotor, as well as, the number of blades

The main/tail rotor, each have their own basic frequency and a number of harmonics/sub harmonics are also generated. There is also the amplitude factor which is quite significant, so some dominant harmonics may become more critical than others

These frequencies are damped out, in theory at least, using various methods. vibrations can lead to fatigue failures

One has seen very significant 5R vibrations from a tail rotor that had to be specifically damped out using critically sited ballast weights.

If the some vibration main/harmonic component cannot be damped out to the required design specified levels, then some critical LRUs etc may even end up being mounted on specially tuned (for that critical frequency, as well as, amplitude level) anti vibration mountings which are normally sealed. These mountings are lifed components, and are to be replaced regularly. Such mountings are like a black box with no real way of knowing what their internal condition is. They are usually replaced as a complete set, even if only one is damaged. Things may be different now.

I have seen the innards of the ALH but that was many years ago. The details escape me now, and besides, there may have been many modifications that would have been done in the years past.

Loose bolts can result from many factors viz mechanical damage during installation/reinstallation, not locked down (there are various methods of locking in use and design specifies which method to be used where), wrong bolt used, cheaper substitutes, different material used in the bolt manufacture and are being used without design, and materials lab clearances itiyadi, itiyadi

The transmission chain is a complex system and also flight critical, and so there would/should be frequent checks to ascertain and verify system integrity.

loose bolts in this chain (or any chain involving aviation systems/sub systems, for that matter) would be totally unacceptable

MeshaVishwas · Post by **MeshaVishwas** » 16 May 2023 16:43

Ilmavoimat Hawk crash-Scramble.nl

Yesterday, 15 May 2023, the Finnish Air Force (Ilmavoimat) Hawk (HW-320) of HävLLv 41 (Hävittäjälentolaivue 41, or 41 Fighter Squadron) crashed in the afternoon south of Keuruu. Both pilots were saved by the Martin Baker ejection seat.

According to the Air Force, both pilots were sent to the hospital for further examination. The Central Finland Rescue Service was alerted about the air traffic accident at 14:15 hours local time, with the location being near Heinäsentie in Keuruu, which is located near the border of Pirkanmaa. About ten emergency services units were called to the scene.

Before the accident, the mishap aircraft (MA) and another Hawk had been conducting a normal air combat exercise as part of the training programme. As the exercise was nearly finished, the MA split off from its partner, due to abnormal vibration in the plane's controls. Soon after, the crew issued a May Day-call, stating they had an emergency.

As a result of the crash the air force has grounded all Hawks and an investigation into the cause has been opened.

So in such a case, will VayuSena ask OEM (BAe) to share findings or will OEM proactively share the findings apply fixes if necessary?

basant · Post by **basant** » 16 May 2023 19:58

chetak wrote:Some landings are precautionary landings and it is made at the sole discretion and professional assessment of the captain of the aircraft.

BTW, it takes big hairy testimonials to do what this captain did (leave the fly past), especially in the full glare of national TV cameras, and in full public view of the crowds watching the fly past, and maybe, with senior govt ministers present in the crowd, to make a precautionary landing at a safe place

Thanks sir for the elaboration, I was not aware of this angle. But am still confused as to how technical fault was established, helo could (or did?) not return to base and yet an emergency landing was ruled out. How is a technical fault driven precautionary landing not an emergency landing? Can you please let me know what is the difference?

basant · Post by **basant** » 20 May 2023 20:02

IAF grounds MiG-21 fighter aircraft fleet pending investigations into last crash over Rajasthan

The Indian Air Force (IAF) has grounded the entire fleet of MiG-21 fighter aircraft till the checks are carried out and investigations into the reasons behind the crash over Rajasthan earlier this month are ascertained.

Three people lost their lives in the crash when a MiG-21 Bison aircraft airborne from the Suratgarh air base crashed over Hanumangarh in a village on May 8.

Zynda · Post by **Zynda** » 21 May 2023 19:27

chetak wrote:
Vibrations in a helo are a very complex phenomena because of the main rotor and tail rotor. These are primarily dependent on the speed of rotation of the rotor, as well as, the number of blades

The main/tail rotor, each have their own basic frequency and a number of harmonics/sub harmonics are also generated. There is also the amplitude factor which is quite significant, so some dominant harmonics may become more critical than others

These frequencies are damped out, in theory at least, using various methods. vibrations can lead to fatigue failures

One has seen very significant 5R vibrations from a tail rotor that had to be specifically damped out using critically sited ballast weights.

If the some vibration main/harmonic component cannot be damped out to the required design specified levels, then some critical LRUs etc may even end up being mounted on specially tuned (for that critical frequency, as well as, amplitude level) anti vibration mountings which are normally sealed. These mountings are lifed components, and are to be replaced regularly. Such mountings are like a black box with no real way of knowing what their internal condition is. They are usually replaced as a complete set, even if only one is damaged. Things may be different now.

Chetak saar...brilliant post...of course fatigue due to vibration!

ramana · Post by **ramana** » 22 May 2023 09:40

Chetak and Zyndda, The stress due to vibrations have to go over the elastic limit for fatigue to become dominant.
The loose nut reduces the preload and looks like is causing the stress to go up. And inherent material properties lead to fracture at a stress riser.

chetak · Post by **chetak** » 22 May 2023 13:02

ramana wrote:Chetak and Zyndda, The stress due to vibrations have to go over the elastic limit for fatigue to become dominant.
The loose nut reduces the preload and looks like is causing the stress to go up. And inherent material properties lead to fracture at a stress riser.

The stresses can be cyclic and and they are cumulative.

The gyan below has been collated from the net.

Fatigue failures and crack initiation and development are really very fascinating subjects.

Because of the vibration mechanics, the HF limits to fatigue cycles are fairly easily achievable hence the need for timely inspections or even better, the replacement of the bolt or whatever, on a flying hour basis

Historically, fatigue has been separated into regions of high cycle fatigue that require more than (about 10 raised to 4 to 10 raised to 8 cycles) to failure where stress is low and primarily elastic and low cycle fatigue where there is significant plasticity. Experiments have shown that low cycle fatigue is also crack growth.

Fatigue failures, both for high and low cycles, all follow the same basic steps: crack initiation, crack growth stages I and II, and finally ultimate failure. To begin the process, cracks must nucleate within a material. This process can occur either at stress risers in metallic samples or at areas with a high void density in polymer samples. These cracks propagate slowly at first during stage I crack growth along crystallographic planes, where shear stresses are highest. Once the cracks reach a critical size they propagate quickly during stage II crack growth in a direction perpendicular to the applied force. These cracks can eventually lead to the ultimate failure of the material, often in a brittle catastrophic fashion.

The formation of initial cracks preceding fatigue failure is a separate process consisting of four discrete steps in metallic samples. The material will develop cell structures and harden in response to the applied load. This causes the amplitude of the applied stress to increase given the new restraints on strain. These newly formed cell structures will eventually break down with the formation of persistent slip bands (PSBs). Slip in the material is localized at these PSBs, and the exaggerated slip can now serve as a stress concentrator for a crack to form. Nucleation and growth of a crack to a detectable size accounts for most of the cracking process. It is for this reason that cyclic fatigue failures seem to occur so suddenly where the bulk of the changes in the material are not visible without destructive testing. Even in normally ductile materials, fatigue failures will resemble sudden brittle failures.

PSB-induced slip planes result in intrusions and extrusions along the surface of a material, often occurring in pairs. This slip is not a microstructural change within the material, but rather a propagation of dislocations within the material. Instead of a smooth interface, the intrusions and extrusions will cause the surface of the material to resemble the edge of a deck of cards, where not all cards are perfectly aligned. Slip-induced intrusions and extrusions create extremely fine surface structures on the material. With surface structure size inversely related to stress concentration factors, PSB-induced surface slip can cause fractures to initiate.

These steps can also be bypassed entirely if the cracks form at a pre-existing stress concentrator such as from an inclusion in the material or from a geometric stress concentrator caused by a sharp internal corner or fillet.

Most of the fatigue life is generally consumed in the crack growth phase. The rate of growth is primarily driven by the range of cyclic loading although additional factors such as mean stress, environment, overloads and underloads can also affect the rate of growth. Crack growth may stop if the loads are small enough to fall below a critical threshold.

Fatigue cracks can grow from material or manufacturing defects from as small as 10 μm.

When the stress intensity exceeds a critical value known as the fracture toughness, unsustainable fast fracture will occur, usually by a process of microvoid coalescence. Prior to final fracture, the fracture surface may contain a mixture of areas of fatigue and fast fracture.

Fatigue is a process that has a degree of randomness (stochastic), often showing considerable scatter even in seemingly identical samples in well controlled environments. Fatigue is usually associated with tensile stresses but fatigue cracks have been reported due to compressive loads.

A quick analysis of the fracture surface of a fatigue failure will often show features casually referred to as “beach marks”. These indicate the propagation of the failure from the initial cracks. Once the crack size has reached a critical level, it will propagate very rapidly until the fracture is complete. Fatigue is a process of the cycle-by-cycle accumulation of damage in a material undergoing fluctuating stresses and strains .

A main feature of fatigue is that the load is not large enough to cause global plastic deformation or immediate failure. Instead, failure occurs after a component has experienced a certain number of load fluctuations, that is, after the accumulated damage has reached a critical level.

In general, the two-stage theory can be used to describe the process of fatigue failure The first stage is the fatigue crack initiation which starts from the surface of a component, where fatigue damage begins as shear cracks on crystallographic slip planes. The second stage is the crack growth which takes place in a direction normal to the applied stress, and eventually allows a fracture to occur.

metal fatigue, is the weakened condition induced in metal parts or structures by repeated stresses or loadings, ultimately resulting in fracture under a stress much weaker than that necessary to cause fracture in a single application.

a structure will fail after a certain number of repeated loading and unloading, so-called load cycles, rather than after one load cycle as simulated in a static analysis.

The grain size of the chosen material plays an important role. As the average grain size decreases, the metal becomes stronger (more resistant to plastic flow) and as the grain size increases, the opposite effect on strength occurs. In general, for a given alloy and thickness, ductility increases with grain size and strength decreases.

Decreasing grain size decreases the amount of possible pile up at the boundary, increasing the amount of applied stress necessary to move a dislocation across a grain boundary. The higher the applied stress needed to move the dislocation, the higher the yield strength.

Fatigue is a process of the cycle-by-cycle accumulation of damage in a material undergoing fluctuating stresses and strains . A main feature of fatigue is that the load is not large enough to cause global plastic deformation or immediate failure. Instead, failure occurs after a component has experienced a certain number of load fluctuations, that is, after the accumulated damage has reached a critical level.

In general, the two-stage theory can be used to describe the process of fatigue failure . The first stage is the fatigue crack initiation which starts from the surface of a component, where fatigue damage begins as shear cracks on crystallographic slip planes. The second stage is the crack growth which takes place in a direction normal to the applied stress, and eventually allows a fracture to occur.

ramana · Post by **ramana** » 22 May 2023 20:26

I did this for a living. Very familiar with high cycle fatigue and low cycle fatigue. Coffin Miner's rule of cumulative damage. Both need stress concentration issues. And low fracture toughness materials. Aluminum has high strength but shows no yield. Hence it is alloyed to give it yielding property.

My question is still how does a push-pull linkage develop low cycle fatigue leading to fracture due to a loose nut?

We are not getting the full story.

and where were these washers installed?
HAL implies not being at their factory.
If not why were these rods re-installed?
IAF is quiet.
Usually when you have a pattern of failures a more senior officer should be put in charge to ensure the right things are looked at and appropriate corrective action implemented.

chetak · Post by **chetak** » 22 May 2023 20:47

ramana wrote:I did this for a living. Very familiar with high cycle fatigue and low cycle fatigue. Coffin Miner's rule of cumulative damage. Both need stress concentration issues. And low fracture toughness materials. Aluminum has high strength but shows no yield. Hence it is alloyed to give it yielding property.

My question is still how does a push-pull linkage develop low cycle fatigue leading to fracture due to a loose nut?

We are not getting the full story.

and where were these washers installed?
HAL implies not being at their factory.
If not why were these rods re-installed?
IAF is quiet.

Usually when you have a pattern of failures a more senior officer should be put in charge to ensure the right things are looked at and appropriate corrective action implemented.

Very seriously doubt if such information will show up on any public forums

ramana · Post by **ramana** » 23 May 2023 06:19

Did they tell CDS the full story?
Thats all that matters.

chetak · Post by **chetak** » 28 May 2023 16:06

ramana wrote:I did this for a living. Very familiar with high cycle fatigue and low cycle fatigue. Coffin Miner's rule of cumulative damage. Both need stress concentration issues. And low fracture toughness materials. Aluminum has high strength but shows no yield. Hence it is alloyed to give it yielding property.

My question is still how does a push-pull linkage develop low cycle fatigue leading to fracture due to a loose nut?

We are not getting the full story.

and where were these washers installed?
HAL implies not being at their factory.
If not why were these rods re-installed?

IAF is quiet.
Usually when you have a pattern of failures a more senior officer should be put in charge to ensure the right things are looked at and appropriate corrective action implemented.

The PSU may have been be doing contract servicing for at least one of the operators involved

Rakesh · Post by **Rakesh** » 29 May 2023 19:25

https://twitter.com/IAF_MCC/status/1663 ... 17984?s=20 ---> An AH-64 Apache helicopter of the IAF carried out a precautionary landing near Bhind, during routine operational training. All crew and the aircraft are safe. The rectification party has reached the site.

https://twitter.com/IAF_MCC/status/1663 ... 84288?s=20 ----> The snag on the Apache helicopter was rectified onsite, following which it has been flown back to base.

VIDEO: https://twitter.com/DefenceDecode/statu ... 75906?s=20 ---> An IAF AH-64E Apache attack helicopter has made an emergency landing in a field in a village of Bhind district of Madhya Pradesh.

Rakesh · Post by **Rakesh** » 31 May 2023 17:46

https://twitter.com/livefist/status/166 ... 65825?s=20 ---> Grounded twice this year, the HAL Dhruv is being ungrounded now only for emergency/urgent ops. Report by @rahulsinghx

Shubham · Post by **Shubham** » 01 Jun 2023 14:29

One Kiran Trainer aircraft crashes during routine sortie in outskirts of Bengaluru.

Crew reported safe as per news.

https://m.timesofindia.com/city/bengalu ... 672543.cms

Manish_P · Post by **Manish_P** » 01 Jun 2023 17:41

Relieved to hear no loss of life... neither the aviators, nor civilians on ground.

Bala Vignesh · Post by **Bala Vignesh** » 03 Jun 2023 00:40

Kiran from bangalore, I am guessing ASTE. Glad to know no loss of life or limb..

Rakesh · Post by **Rakesh** » 03 Jun 2023 00:46

https://twitter.com/realkaypius/status/ ... 89504?s=20 ---> Successful ejection with no lives lost from one of IAF's oldest trainers is good news; the CoI will deal with evidence & data. The first female aircrew from the Indian military to eject IIRC? AVM Tejpal Singh, ETP, & Squadron Leader Bhumika, FTE, are both safe & recovering.

Tanaji · Post by **Tanaji** » 05 Jun 2023 03:48

Genuinely curious: why do an AVM and a squadron leader feel the need to fly a trainer such as Kiran?

sajaym · Post by **sajaym** » 06 Jun 2023 18:18

Tanaji wrote:Genuinely curious: why do an AVM and a squadron leader feel the need to fly a trainer such as Kiran?

The AVM seems to be ETP (E? TEST PILOT) and the Squadron Leader (https://www.google.com/amp/s/www.hindus ... 4-amp.html) seems to be an FTE (Flight Test Engineer), so both were probably on some kind of evaluation test flight.

chetak · Post by **chetak** » 06 Jun 2023 18:55

sajaym wrote:
Tanaji wrote:Genuinely curious: why do an AVM and a squadron leader feel the need to fly a trainer such as Kiran?
The AVM seems to be ETP (E? TEST PILOT) and the Squadron Leader (https://www.google.com/amp/s/www.hindus ... 4-amp.html) seems to be an FTE (Flight Test Engineer), so both were probably on some kind of evaluation test flight.

experimental

The other type is the PTP, the production test pilot