Autopilot bug on scoot 787

So I was doing a flight on a scoot 787-9 from Sydney to Singapore and about half way in the flight while I was cruising the autopilot speed switched and showed my speed in mach then Suddenly the plane stalled and I had to try about 3-4 times before I got it working again. Is this some sort of bug? **Iphone 7**: **iOS 15.8.2**:

Your speed will switch from speed to Mach when you hit Flight Level 28,000.

Stalls usually occur when you are too heavy for the Vertical Speed and/or the initial Cruise Altitude you have set, also you’ll need to burn some fuel before you reach your final cruise altitude which requires the use of Step Climbing. Example, your initial cruise will be between 28,000 and 32,000, but as you burn off fuel, you be able to reach higher altitudes one step up at a time until you reach 39,000. This Step Climb method is used for long haul and ultra long haul flights.

Check out this Long Haul Tutorial, it’s a great read:

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@F1_Boy Welcome to the community!

In addition to what Levet said, some key points (don’t feel like you have to take it in all at once, just consider as a possible reference for what’s what):

There are two kinds of speed limits: ATC related (traffic safety) and “flyability” related (keeping the aircraft flying, or aerodynamically safe)

IAS (indicated airspeed) and mach speed relate to “flyability.”

IAS can’t be too high (you damage the aircraft structure)
IAS can’t be too low (you can’t stay in the air)

Mach can’t be too high (you can’t get too close to the speed of sound: the wings will misbehave and, again, you can’t stay in the air)

Mach only comes into play at higher altitudes* (the reason for the switch-over as you climb); IAS is always important (if you’re too slow high up or low down, you can’t stay in the air)

The quick summary for step climbing (translation: not flying higher than your weight allows) is: If safe IAS cannot be maintained, your VS (vertical speed) is too high.

Keep lowering your VS until your IAS can be safely maintained.

This tells you when you have reached the current top of your step climb: when you get high enough that VS has to be about zero to keep your IAS from deteriorating.

*Mach is measured in TAS (true airspeed), and TAS gets faster and faster compared to IAS as you climb into thinner air.

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