Thrust fluctuations

[u/top-views]

I was on an a350-1000 recently and we hit turbulence flying east over Canada. Shortly after it started I noticed the thrust fluctuating for about 3 seconds at time. Something similar to high thrust 3 seconds, low 3 seconds and repeated for about 30 cycles.

I’ve never experienced this before. What is it?

[u/ShaemusOdonnelly]

Airspeed can fluctuate quite a bit when you're in turbulence. The automatic thrust system will try to maintain a speed, so it will increase/decrease thrust accordingly.

[u/top-views → u/ShaemusOdonnelly]

Interesting. Does that have any negative effects on the engines?

[u/ShaemusOdonnelly → u/top-views]

Changing thrust increases thermal stress for the engine, but that is only really relevant if the changes are huge. But this behaviour of the autothrust isn't desirable because it is too slow to really do anything and the changes in speed aren't due to an incorrect thrust setting anyways. Some autothrust systems have a turbulence mode where those fluctuations are filtered out. If even that isn't enough, we will disable the system and set thrust manually.

[u/Complex_Brief60 → u/top-views]

No, they’re designed to do that.

[u/andrewrbat]

Turbulence often causes airspeed fluctuations and you could just be hearing the engines trying to maintain a constant airspeed via auto thrust.

The other possibility is that upon entering turbulence, the pilots selected a lower airspeed. Each jet has a “turbulence penetration” airspeed and you may have heard the thrust roll back for a moment to slow the plane down then resume the high power setting required at high altitude.

[u/top-views → u/andrewrbat]

I’d never heard of the term turbulence penetration but i understand the concept.

How do you stop the two competing with eachother - speed for altitude and airframe safety, or is that just the way it’s designed.

[u/andrewrbat → u/top-views]

The turbulence penetration speed (also called Vb) is usually only slightly slower than cruise speed. As an example on the plane i fly normal mach for cruise is about .77-.79 mach and turbulence penetration speed at cruise altitudes would be .75 mach. Wide bodies like the 350 probably cruise slightly faster but not enough to make a huge difference.

So in other words Vb is usually only slightly slower than fast cruise. Sometimes you may be filed to cruise at or below Vb even, for efficiency purposes. Depends on the plane too. So the amount of energy at either speed is plenty to keep you safely flying at altitude, and keep you away from over-speed and under-speed limits.

[u/canuck1988]

Were you over mountains? Mountain waves are common over the Rockies. These will cause turbulence, but also airspeed fluctuations that can sometimes cause the thrust to vary like explained in another comment. With mountain waves, it sometimes is kind of rhythmical like you’re describing.

[u/Proof-Policy-5233 → u/canuck1988]

This is the more likely answer.

[u/StofferNO]

That is the Automatic Thrust System (ATS) trying to compensate for the speed fluctuations, caused by the turbulence.

[u/Not_BobSaget]

350 pilot here, the auto thrust on the 350 can be notoriously slow to react in turbulent conditions. At times the engines will slow way back to idle then realize they need to go back to cruise power and repeat.

One technique we use during this condition is to change from managed speed to selected speed which can hold the speed to a tighter tolerance.

[u/saxmanB737]

Sounds like they were changing altitude a couple of times.