Intresting article about MiG-27's
GSh-6-30 cannon.
However, another serious problem appeared with the aircraft gun mounting: despite the reduced ballistics [Ed: from the use of shorter barrels] the recoil force of the GSh-6-30A was about 5,500 kg. The impact loads caused by firing were very high for the aircraft to absorb, particularly as its structure was a development of a light fighter. For ground firing tests the gun was initially mounted on a wooden testbed, but at the first trial firing of the "Shestistvolka" the testbed simply fell apart.
Further problems occurred during the first air firing tests. It was discovered that the impact and frequency characteristics generating by firing the GSh-6-30A on the ground did not correspond to those which took place in the air. The first 25-round burst made in flight was ended by the failure of all of the avionics in the cockpit. In further test flights there were cases of deformation and even tearing away of the nose undercarriage door, and because of the strong vibrations the ammunition feed fell apart. Electronic equipment in an aft-of-cockpit compartment also failed.
To reduce the influence of gun firing on the fuselage, the axis of a gun was inclined downwards 1°13 '.
In combination with its sighting system the GSh-6-30А was very accurate. In Soviet Air Force regiments gun firing was often exercised during training flights. Usually the ammunition belts alternated two OFZ rounds with one BZ, but on examination flights each fourth or fifth round was a tracer. Firing was possible in a manual mode, with "approximate" targeting using the fixed grid of a sight. In automatic aiming mode the PrNK-23 navigation system was used. It gave the necessary corrections and lead on an S-17VG sight together with information about the target and effective firing range. When detonating on impact with the ground, the shell fragments scattered up to 200 metres in the air and represented a serious danger to the aircraft. In short 40-round bursts, lasting a fraction of a second, the gun sent to the target about 16 kg of "effective load". In training flights, the gun was loaded with only 30-40 cartridges, sufficient for improving shooting skills.
As one of the measures to tackle the problems, the firing rate was limited to 4,000 rpm. Despite various adaptations and structural reinforcements, the risks involved in firing the gun had to be taken into account during operations. The general view of the aviation regiments was that bursts of 30-40 rounds had no serious consequences, but a long burst of 2.5-3 seconds was accompanied by "crunches and crashes". Apart from other advantages, restricting shooting to short bursts extended the effective life of the gun.
In August 1980 a Major Shvyrev, of one of squadrons of the 722nd bomber regiment, was returning from a gun-firing flight when he discovered that the doors of the forward landing gear were deformed and it was not possible to lower the gear. The pilot had to land his aircraft on the two main wheels, for the first time during the service of the MiG-27 in the VVS. Despite this, the aircraft suffered no serious damage and soon flew again. Another case occurred on August 8th 1988, when a MiG-27D of Lieutenant M.V.Poluektov, of the 19th bombing regiment based in East Germany, returned from firing practice with the GSh-6-30. He also found that the forward gear was not down because of jammed doors. As was observed later in the report, "the pilot had high moral-political preparation" and was able to land the aircraft on the main wheels with minimal damage. A similar case with more serious consequences occurred in the 599th bombing regiment on May, 15, 1990. During firing a muzzle attachment broke, and this became jammed against the nose-gear doors, so the nose wheel could not release. The MiG-27K ploughed the runway with its nose, and had to be written off [Ed: the pilot obviously lacked something in "moral-political preparation"]
The avionics systems often became turned off because the electrical commutating switches became disconnected. On April 18th 1988 a MiG-27 from the 24th Air Division arrived at an air base without its communications or navigation systems working: during firing, the navigation, radio and gyroscopes had switched off. In East Germany on 2nd September 1989 a MiG-27 suffered full radio communication failure; contacts in the radio had broken and printed circuit boards had cracked. In aircraft of the 23rd Air Army in January 1989, gun shooting resulted in two incidents. In the 58th bomber regiment a broken electrical safety fuse caused almost complete failure of the stabilizer, flaps, landing gear and navigation system control. One week earlier in the 266th bomber regiment, a MiG-27K returned from a flight without the cockpit canopy: emergency locks had released the canopy while shooting.
Even given this background, the case which occurred in the 24th Division on 29th March 1989 appears to have been unique. On recovery from a dive after gun shooting, the instrument panel fell onto the pilot's legs: the panel fastenings had been sheared by the recoil force. The pilot reached an airbase holding the panel, which was hanging via electric cables, by one hand.
It frequently happened that the recoil force of firing broke the reflector sight. Landing headlights broke so frequently, that before flights involving gun firing, they were removed and replaced with caps. The introduction of protective deflectors did not completely rescue a situation which demanded changes to the "MiG-27 Pilots Instruction": after night shooting, landing took place only on runways illuminated by floodlights.
A continuous burst of fire was dangerous because of the risk of barrel overheating. That could lead to the explosion of primers and cartridges, and even of shells. Each such case was considered as extraordinary and was examined by the OKB and Tula TcseKB. In the 16th Air Army such an incident on 22nd January 1990 resulted in the loss of a MiG-27K. The detonation of a shell in the barrel of the GSh-6-30A broke the gun apart. Fragments of the gun damaged a fuel tank, electric cables and a hydraulic system. The explosion of an oxygen tank "instantly inflated" a fire. The pilot lost control and ejected from the burning airplane. A similar case took place three months later on a firing range in Lunitcsa with a MiG-27K from the 39th Division. Some shells exploded near the nose, but the aircraft managed to return to base despite the shell-holes, broken hatches and glass housing of the sighting system, and dents on the compressor blades.
This type of "self-inflicted wound" happened even on the ground, usually the result of an oversight by the ground crew. As a protective measure a "ground/air" electric circuit, activated by weight on the landing gear, prevented the gun from firing while the plane was on the ground. Sometimes during flight preparations this interlock was disconnected or overlooked, so that the gun was armed as soon as the nosewheel left the ground. When parallel tests of different systems to check weapon control took place, this could result in the gun firing. In 1983 at the 236th bomber regiment in Chertok, such an accident demolished the forward landing gear, and a similar case took place in the 88th bomber regiment in Suurukul. In Lipetsk, on September 2nd 1986, only one cartridge remained in a MiG-27D's gun as it was returning from a flight. After landing, the gun fired and hit the nose gear, igniting the flammable AMG-10 hydraulic fluid which was contained in nose gear under high pressure.
With the retirement of the MiG-27 from Russian Air Force service, the GSh-6-30 guns were also withdrawn.
Cheers....
