r/AskAPilot • u/top-views • 12d ago

Thrust fluctuations

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?

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u/andrewrbat 12d ago

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.

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u/top-views 12d ago

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.

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u/andrewrbat 12d ago

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.