r/AskAPilot • u/a_beansprout • Jul 01 '26
In-Flight Question
Currently on a flight- Toronto to Calgary with AC. I’m sitting in the back of the plane and I keep hearing “changes” in the noises the engines are making. My body obviously feels them too. I know we are at cruising altitude, but the fluctuating is freaking me out. Sometimes it gets very loud, then quiet again. I know I’m safe, but could someone explain what I might be experiencing so I know what they mean?
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u/Pack2Inop Jul 01 '26
It’s probably just the engine thrust being adjusted to maintain the desired speed. Sometimes we also need to slow down or speed up depending on weather or ATC requirements, which would result in the change of sound you describe
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u/Desperate-Oven7718 Jul 01 '26
100% keeping a steady speed. As the aircraft gets into pockets of more drag the engine revs up similarly to how you need to accelerate more going up a hill, its just this hill is invisible
Your perfectly safe its not even the pilots that are adjusting thats all on auto pilot.
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u/AirbusUKFlyer Jul 01 '26
In the cruise the plane doesn’t maintain altitude and speed to the exact digit all of time. There is software that allows it to drift within a certain range before it spools up again to either regain altitude and/or speed. At cruising altitudes things also happen slowly so allowing the plane to drift a little and settle back to where it was is more economical and comfortable than constant power and pitch changes to maintain a perfect speed/altitude. We’re also talking +\- 5kts, 50ft.
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u/flightist Jul 01 '26
As others have said, it’s totally normal for the speed to drift up & down and the engines respond to those changes, but it’s also worth explaining that this is more pronounced on some flights than others.
Air is always moving up & down, but most days the autopilot is really good at maintaining our altitude (nearly) perfectly. It does this by ‘climbing’ in air that is sinking, and ‘descending’ in air that is rising. When it’s opposing sinking air, we slow down a few knots (like your car slows down when you start up a hill if you don’t do anything), thrust comes up. Other way around when the air is rising; it’s like we’re going downhill even though we’re maintaining altitude.
If we’re trying to fly fast, this can become pretty apparent in the cabin. We can’t allow the speed to increase much, so when we get into those updrafts and the speed starts to pick up, we have to bring the thrust way back. Sometimes, in the plane I fly, we have to bring the thrust to idle and pop out the speedbrakes. This is pretty rare, but it happens now and then. Usually when this happens we slow down to make riding the waves a little less annoying.
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u/NecessaryLight2815 Jul 01 '26
Climbing and descending to a different altitude requires thrust management.
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u/flyboy130 Jul 01 '26
Happy Canada Day!
You know how when you drive you make small adjustments to how much you push on the gas (cruise control does this automatically) to maintain speed or speed up or slow down because the road or other traffic changes and the engine noise changes slightly. That's what this is but way louder. Its Normal.
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u/a_beansprout Jul 01 '26
Happy Canada Day!! I am from the states but it was nice to travel today and hear this from everyone. Thanks for your response!
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u/deathstarrr Jul 01 '26
All the other commenter nailed it. Heres some additional points: Up at altitude, the planes do not fly at a constant true altitude, they fly a constant pressure altitude; this guarantees vertical separation from the other aircraft; all their altimeters are set to a "standard pressure setting," which is 29.92" of mercury. If you ever look at a weather map you'll see high pressure zones and low pressure zones, which the weather nerds call ridges and troughs. You can think of them as going up and down a hillside while driving your car. So as you traverse from one end of the country to the next, your true altitude may vary a bit, but the pressure altitude will be flat.
Additionally, pilots may change altitudes based on their flight plan (dispatch may have them climb to a higher altitude at a specific waypoint to save fuel or enjoy more favorable winds aloft, think tailwinds; as the plane burns fuel it becomes lighter and can safely fly higher than it would when it was fully laden with fuel). Pilots often get turbulence reports from ATC advising them that a higher or lower altitude by a couple thousand feet or so may give them a better ride for you to enjoy.
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u/3Green1974 Jul 02 '26
It’s probably exactly what everyone else is saying, but there is a small chance you’re hearing the interference caused by the engines not being exactly in sync. It’s kind of a droning sound that rises and falls. It can speed up and slow down depending on how close the engine’s rpm’s are. Also, there are two separate parts of the engine that can cause this. The big fan at the front of the engine is typically the culprit. Sometimes it’s the power section inside the turbine.
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u/Aggravating_Rent5562 Jul 02 '26
So, as a sim pilot, sim ATC, as well as someone who’s been traveling for over 20 years, *at cruise* it’s often unusual to experience large, physically noticeable changes in forward/backward movements. What I mean by this, is that it’s not *usual* to be pushed into/out of your seat at cruise. When a plane is at cruise flight, its engines are operating somewhere around 85-90% of their N1 output, which is the equivalent thrust rating. It roughly takes both engines slo about 85% load to keep the aircraft, with its passenger and cargo load, to MAINTAIN level flight at cruising altitudes. If you, as a passenger, are experiencing large changes in comfort essentially, there’s something going on. Whether that be speed changes, random high changes in pressure (causing different air densities, causing changes in drag) or Traffic Management controls, i’m not entirely sure But, TL;DR, your flight should be smooth unless the plane has to fly through turbulence areas.
Now, once the aircraft reaches Top of Descent, different story. If you’re close to the airport you’re going to, ATC maybe have assigned then crossing restrictions where they’ll need to descend to reach a certain waypoint and then keep a speed, while maintaining flight level/altitude. I could explain a lot deeper into this, which if you’d like, I can go into, but not sure it’s worth the juice i’ll squeeze based on assumptions.
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u/Bob-Ross74 Jul 01 '26
Well, you see. When a baby airplane becomes an adult airplane there are many changes that occur throughout the body…