A319/A320/A321 Cruise Speed Oscillation

    I’ve noticed a recurring issue with the A320 family (A320 and A321, possibly the A319 as well).

    During cruise, the aircraft sometimes struggles to maintain a stable target speed. For example, when set to 230 knots, the speed drops significantly (around 224 knots), then increases above the target, and continues oscillating in a repeating cycle.

    It feels like a continuous speed hunting behavior. It doesn’t happen every flight, but I’ve experienced it multiple times, especially with the A321.

    Device: Galaxy M35, Aerofly mobile.

    That's perfectly normal, it happens even in real life, there's nothing wrong with it.

    This also needs to happen on Boeing aircraft, and there also needs to be a way, like on Airbus aircraft, when setting a Cost Index, to increase or decrease the speed, but unlike the B737-800 which only stays at Mach 0.789, it's much better. That's too generic.

    You can see it's at Mach .77, but it went a little further to .774. Then the management system sees that it's a bit fast, so the engine downshifts and the speed drops even further from .77.

    Edited once, last by Jet-Pack (IPACS): Merged 2 posts created by Von Jack into this post. (February 19, 2026 at 1:32 PM).

    Quote from Von Jack

    This also needs to happen on Boeing aircraft, and there also needs to be a way, like on Airbus aircraft, when setting a Cost Index, to increase or decrease the speed, but unlike the B737-800 which only stays at Mach 0.789, it's much better. That's too generic.

    Thank you for the respectful response. Small variations are indeed normal, however the variations I mentioned are quite excessive. You can clearly hear the engines increasing and decreasing thrust repeatedly.

    Instead of the system adding just a small amount of thrust to recover speed, it adds a lot of power, and once the speed exceeds the target it reduces thrust too much again. This creates an infinite oscillation cycle that becomes quite noticeable and somewhat annoying.

    Quote from Davyy11

    Thank you for the respectful response. Small variations are indeed normal, however the variations I mentioned are quite excessive. You can clearly hear the engines increasing and decreasing thrust repeatedly.

    Instead of the system adding just a small amount of thrust to recover speed, it adds a lot of power, and once the speed exceeds the target it reduces thrust too much again. This creates an infinite oscillation cycle that becomes quite noticeable and somewhat annoying.

    That's how it is in real life too; in the image, maybe the speed was still increasing or already decreasing, but it can increase or decrease by about 7 knots from the managed speed.

    Quote from Von Jack

    That's how it is in real life too; in the image, maybe the speed was still increasing or already decreasing, but it can increase or decrease by about 7 knots from the managed speed.

    I understand that small variations (even up to 5–7 knots) can happen in real life, especially in VNAV or with wind changes.

    What I’m describing is more of a continuous overcorrection cycle, with clearly noticeable thrust increases and reductions in stable cruise conditions. It feels less like normal fluctuation and more like aggressive speed hunting.

    That’s why I believe it might be a tuning issue rather than realistic behavior.