Per-minute description: before the crash the Il-112v engines worked in a dangerous mode

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Per-minute description: before the crash the Il-112v engines worked in a dangerous mode
Per-minute description: before the crash the Il-112v engines worked in a dangerous mode
8 September, 19:06IncidentsPhoto: Фото: e-news.su
An intermediate landing of the plane for only five minutes could play a fatal role in the operation of its engines.

Andrey Zlobin

As new information appears on aviation resources, additional assumptions can be made about the causes of the Il-112v plane crash. So on the portal "Aviation of Russia" there was a minute-by-minute description of the events associated with the flight of the transport aircraft. These data clarify my earlier point of view, including regarding the organizational reasons that caused the engine malfunction. First of all, attention is drawn to the ill-conceived flight program. If the minute-by-minute data is correct, then it can be strictly proved that the Il-112v engines worked in a dangerous mode before the crash. In this case, it does not matter at all what aircraft and what engines we are talking about. For any aircraft gas turbine engines (GTE), there are general ideas about the modes in which malfunctions may occur.

There is a mode of operation of an aviation gas turbine engine, which is called "repeated throttle response". In aviation practice, there are cases when an obstacle suddenly appears on the runway before landing an aircraft, another aircraft, a car or people, and the pilots are forced to go around. And even if the plane has already touched the ground, the engines are immediately switched from “flying idle” mode back to takeoff mode. In the language of engine engineers, this is "repeated throttle response". In the process of repeated throttle response, the different thermal inertia of the thin engine body and its massive rotor (turbine and compressor disks) are clearly manifested.

This difference in thermal properties can cause a dangerous reduction in the gap between rotating and stationary engine elements, up to their touch. There is a danger of an emergency. For well-developed, serial aircraft engines, trouble-free operation is guaranteed. For this purpose, the most thorough computational studies are carried out and numerous long-term tests are carried out, both at the stands and in flight.

The estimated diagram of the modes on which the IL-112v engines operated before the disaster: NOM - nominal, PMG - flight idle, ZMG - ground idle, VTL - take-off (maximum)

If we analyze the flight program of the Il-112v dated August 17, then its dangerous resemblance to “go-around” and the operation of the engines in the mode of repeated acceleration is striking. It turns out that after landing in Kubinka, the plane was on the ground for only 5 minutes, after which it immediately took off again. This is not enough to fully compensate for the thermal inertia of the rotor and stator (housing) of the motor. Moreover, cases of engine overheating were recorded in previous flights.

I repeat, if we were talking about a production aircraft and engines, then there would be no complaints about the flight program. But the aircraft and engines were in the testing stage, and it was clearly not reasonable to subject them to such a rigid "comb" of modes. The mode comb is shown schematically in the diagram. Probably, the plane could fly from Zhukovsky to Kubinka at the nominal mode, then - landing (flying low throttle), then - ground low throttle, again takeoff mode and the passage at nominal.

Variations are possible, but the main thing is beyond doubt - for engines in the test stage, installed on the wing of the tested aircraft, such a leapfrog of modes is extremely dangerous. It would be much safer if the plane made the passage at the end of the flight from Zhukovsky without making a stopover. It was also safer not to take off that day after successfully landing in Kubinka. Probably, there was still a reason why the Il-112v landed in Kubinka for a few minutes. These strange few minutes, most likely, played their fatal role. I think the experienced test pilots understood what a great danger a short landing and immediately after that a re-takeoff was.

Andrey Zlobin - candidate of technical sciences; in the 90s the head of the motor sector of CIAM (Central Institute of Aviation Motors).

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