Mountain Wave Causes A320 Engine Roll Back

[Guest]

Here is the goto to the "Avherald" article. 2008-2019 by The Aviation Herald, all rights reserved, reprint and republishing prohibited,

quite a discussion follows the report, I see in the comments that this was the 2nd A320 mountain wave engine roll back .

http://www.avherald.com/h?article=4c3a18ab&opt=0

The goto to the Dec report is: http://www.avherald.com/h?article=4c2471b6&opt=0

[blues deville]

Mountain wave can be nasty and most times just annoying but an added engine roll back certainly increases your attention to the event. I’ve found the best technique in continuous mountain wave is to disconnect the auto thrust and set an average safe power setting. Having an autothrust system chase constant speed changes burns up extra fuel and can sometimes roll way back in its attempt to manage the unstable conditions. 

[Guest]

And it is now in the news:  https://www.flightglobal.com/news/articles/a320-suffered-loss-of-thrust-as-it-entered-mountain-455498/

A320 suffered loss of thrust as it entered mountain wave

 

04 February, 2019

  | SOURCE: Flight Dashboard

  | BY: David Kaminski-Morrow

Air Canada is probing an incident in which an Airbus A320 lost thrust on one engine as is passed through a mountain wave en route to Los Angeles.

The aircraft, with 146 passengers and five crew members, was operating from Toronto on 27 January, says the Transportation Safety Board of Canada.

It says the A320 (C-FLSS) was cruising at 36,000ft some 85nm southwest of Colorado Springs, in the Rocky Mountains region.

The aircraft passed through a mountain wave, oscillations in the atmosphere caused when air current flow is disturbed over high terrain.

Its left-hand CFM International CFM56 powerplant rolled back, says the board, from 90% of N1 to 60%.

 

"The aircraft started to lose speed, and started to [descend]," it adds. The crew informed air traffic control of the situation, requesting a change in cruise altitude to 30,000ft, but no emergency was declared.

 

While the left-hand engine showed "slight vibration", says the board, there was no alert from the aircraft’s centralised monitoring system. The engine recovered shortly afterwards and the aircraft landed without further incident.

 

Air Canada is assessing the event in an effort to establish the cause, says the safety board.

[Don Hudson]

A mountain wave "causes" an engine roll-back?

What am I missing?

[anonymous]

I suspect that as the speed fluctuated, the autothrust brought the speed back to flight idle and one of the engines rolled back further....just guessing.

 

The 767-200s at AC  powered by the JT9D also could have problems with roll back in the cruise, in similar scenarios

 

[rudder]

I saw a pilot pull thrust back to idle in cruise to avoid barber pole in response to wave on a 737NG. Boeing recommendation on the engine was NOT to do that as it may not respond promptly to subsequent input for normal thrust. Recommended speed brake input.

[blues deville]

2 hours ago, rudder said:

I saw a pilot pull thrust back to idle in cruise to avoid barber pole in response to wave on a 737NG. Boeing recommendation on the engine was NOT to do that as it may not respond promptly to subsequent input for normal thrust. Recommended speed brake input.

Potential overspeed? Correct. Use speed brake. Continuous moderate mountain wave is another issue.  

[Don Hudson]

Understand thrust being reduced either manually or by the autothrust system - experienced it - once right back to idle then slowly up again, also seen it in data but the headline of this article and thread leaves one believing that the aircraft suffered an engine problem because of "mountain wave". Thrust coming back on the A320 due speed is the way it, (or the pilot) is supposed to work. An uncommanded, non-recoverable engine roll-back which does not respond to thrust lever movement is an internal, mechanical issue which has nothing to do with winds, temperatures or pressures outside the airplane.

On a temporary basis the A330 required the engine anti-ice be ON just prior to descent due to thrust reduction scheduling causing internal pressures to change such that the blades could potentially stall. Placing the anti-ice on relieved some of the pressure while a fix was engineered. I don't believe this was ever a requirement on the A320.

 

[boestar]

one also has to be aware of Core Lock issues.  Mor common on the small GE engines on the CRJ aircraft.  The case and the blades cool at different rates.  Rolling back the throttle to idle can cause the case to contract and result in the blades rubbing on the case liner.  The tolerances are VERY tight.  This will cause the engine to not respond to throttle increase.  I am not sure if its an issue on larger engines but its possible.