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TheFly
11-12-2017, 08:29 AM
Russian investigators have detailed a serious loss-of-control incident last month involving a Utair Boeing 737-500 on approach to Moscow Vnukovo.

Federal air transport regulator has revealed that the aircraft was subjected to excessive pitch – up to 45° – and bank of 95° before the crew regained control.

The aircraft (VQ-BJP) had been following a precision approach to runway 06 after a service from Krasnodar on 13 October.

Rosaviatsia states that conditions were overcast, with a cloud base of just 60m, and that the aircraft was in cloud and "out of sight" of ground.

The aircraft was following an arrival pattern which involved passing the IBTER waypoint, south-west of the airport, at a height of 600m before turning right to align with the runway and descending to 400m by the final approach point.

Rosaviatsia says this turn and descent were conducted with the autopilot and autothottle engaged, the landing-gear deployed, and the flaps set at 15°. The engines were operating at 35-40% of their N1 level.

After the turn the aircraft levelled at 400m but its pitch started increasing to 10° and airspeed declined to 133-135kt. The autothrottle raised the thrust level to 75% of N1.

As the 737 approached the entry point to the glidepath, at around 130kt, the crew extended the flap setting to 30° and the autopilot disengaged.

Rosaviatsia says the engine thrust setting had increased further by this point, to 95% of N1, and the power setting – combined with an increasing pitch of 19° – resulted in the jet entering a "smooth climb", while the airspeed declined to 128kt.

The pilot's control column, it states, was pushed nose-down and the thrust levers were pulled back to a lower power setting, but without a disconnection of the autothrottle.

Pitch continued to increase to a maximum of 45° and airspeed bled away to less than 100kt, triggering a stick-shaker alert. The inquiry says the control column registered "alternating deflection" and the aircraft experienced banks of up to 95°.

After the jet reached the maximum pitch the control column was pushed forward and remained this way until the aircraft emerged from the upset.

It had climbed to a height of 750m during the event and, once the crew regained control, the aircraft executed a go-around at 350m. The second approach was uneventful.

Rosaviatsia says that, despite the excessive operating parameters, the aircraft did not breach loading and speed limits for its configuration. None of the 111 passengers and five crew members was injured and the jet was undamaged.

Investigators are still analysing the circumstances of the incident which took place in daylight at around 09:17.

https://www.flightglobal.com/news/articles/serious-utair-737-upset-involved-excessive-pitch-and-442854/

Absolutely amazing this didn’t end up as a smoking hole in the ground.


JamesNoBrakes
11-12-2017, 10:05 PM
Yeah, saw the FDR recreation. Amazing it didn’t turn out worse.

rickair7777
11-13-2017, 10:46 AM
Did it look like automation malfunction?


badflaps
11-13-2017, 11:32 AM
Something really doesn't smell right.

FlyJSH
11-13-2017, 03:24 PM
Yeah, saw the FDR recreation. Amazing it didn’t turn out worse.

Was it this one?

Accident: UTAir B735 at Moscow on Oct 13th 2017, loss of control recovered (http://avherald.com/h?article=4b01e302&opt=0)

Allegheny
11-14-2017, 08:03 AM
I have experience with the 737 but only the 400. However if the aircraft in question has the same autopilot, there is an issue that may be involved.

The Boeing 737 introduces a nose up trim bias at 400ft radio altitude. The system is fail passive and uses this as a safety feature. If the auto-pilot should disconnect below 400 ft due to a fault, the natural tendency of the aircraft will be to start a climb away from the runway. The additional back trim is also useful in the flare mode of the auto-landing process. Gotcha is in attempting a go-around manually after the auto pilot has introduced this trim. (following taken from Eric Parks notes at US Airways in 2002)


Remember that an Autoland uses an Auto Go-Around, you should ONLY press the TO/GA switches to initiate an Auto Go-Around. At about 400 ft. the autopilot will greatly increase back trim on the stabilizer to ensure the nose will come up in the case of autopilot disconnect. If you disconnect to go-around you will have a huge amount of nose up trim to deal with (bad thing!).

In short, DO NOT DISCONNECT the autopilot intentionally to go-around or to land. The system is designed for an Auto Go-Around and Autoland only. If the Captain sees the runway for landing just call LANDING and let the autopilot continue to land. If the Captain does not see the runway just press the TO/GA switches and begin the go-around procedure on autopilot and autothrottles. Autopilot should be disconnected AFTER landing (Auto throttles will automatically disarm after landing).
If this incident crew had forgotten about this then they could be "along for the ride", as the aircraft has a very strong pitch up tendencywith the application of full power.

FlyJSH
11-14-2017, 07:03 PM
For us ignorant types...

The autopilot trims in what would be several pounds of push to nose down to maintain glide path? That is, if everything kicks off, the PF must push hard to maintain level flight?

rickair7777
11-14-2017, 09:15 PM
For us ignorant types...

The autopilot trims in what would be several pounds of push to nose down to maintain glide path? That is, if everything kicks off, the PF must push hard to maintain level flight?


It pre-trims nose up so that when/if the AP is disconnects in the landing realm with thrust idle you don't get a big nose-down excursion.

If you do a go-around in that condition, you get large additional pitch up moment from the engine thrust (under wing engines).

vessbot
11-15-2017, 01:34 PM
At about 400 ft. the autopilot will greatly increase back trim on the stabilizer to ensure the nose will come up in the case of autopilot disconnect. If you disconnect to go-around you will have a huge amount of nose up trim to deal with (bad thing!).[/I]

In short, DO NOT DISCONNECT the autopilot intentionally to go-around or to land. The system is designed for an Auto Go-Around and Autoland only.

Christ, and Airbus gets a rap for designs that take the pilot too much out of the loop...



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