Overspeed Limit keeps dropping in the 777-200ER

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#1

Hello Community,

I recently purchased the 777-200ER and I love it. Only downside is that the speed limit keeps dropping. One minute it’s 330 knots and a few minutes later it goes down to 290 knots. Is this a bug or on purpose?

Device: Galaxy S7
Latest version of IF
Android 6.0

#3

If you’re flying at 28’000 feet at mach .85 you will go at around 320 knots. If you go at 38,000 at mach .85 you will go around 240 Knots, it is science the higher the altitude the lower the oxygen so the engines dont take allot of air :) and its every plane by the way not just the 777.

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#4

Not really much to do with ‘oxygen’ more with density I think you’ll find.

The pitot system for Indicated Airspeed uses the velocity of air ‘particles’ entering the tube to give the ‘pressure’ related to the aircrafts speed. The ‘static’ system is used to compensate for the aircraft altitude. Temperature is corrected using a barometric spring.

What is tricky to compensate for is the ‘density’ of the air. As you climb at a fixed groundspeed the airspeed will seem to reduce as the density of the air reduces thus reducing the ‘pressure’ on the pitot system. A very simplified over view but it gets the point across.

Hence when at 40,000’ plugging along at Mach 0.84 and a ground speed of 550kts your ‘indicated’ airspeed will be in the region of 240-250kts 'ish due to the lack of air density. The system is limiting your indicated airspeed to the max allowable airspeed at the altitude you are at as you will be approaching supersonic flight!

Transition to mach numbers at about 20,000’ and fly a constant Mach 0.84 and your overspeed warnings will disappear as the Mach is a speed relative to the local speed of sound at the altitude you are at.

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#5

It’s airspeed. The higher the altitude, the airspeed would be lower due to the thin air 😊

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#6

I know I love the 777-200ER But that is a problem

#7

So, the higher I go, the slower I go? Is that what you are saying? Also, I fly the 787 and never got this issue before :)

Sorry for not getting this

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#8

Although your Airspeed is dropping, but your GS is still increasing 😊

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#9

Here is a thread with a good discussion and explanation of Ground Speed vs Airspeed. There are several others on the forum too that explain this in detail.

#10

Thanks for letting me know. So if I’m flying at 36.000 ft and my air speed is around 240knts, my ground speed is much higher, correct? So I’m still moving faster than 240knts? Also, how fast is that? And how quickly would I get, for example, to JFK from London?

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#11

Yeah, 250 knots Airspeed in FL240 is slower than 250 knots Airspeed in FL270 and vice versa

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#12

Ok. So if I was at 36,000 ft and at 240 knots, how long would it take to get from London to JFK? Just out of curiosity.

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#13

I’m not sure about that as I’m not an expert about the time calculation.

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#14

It depends on the headwind/tailwind, route, and temperature. There’s no one answer.

#15

What about a rough calculation? Average winds, temperature etc

#16

The max airspeed changes once you cross FL280. Try keeping airspeed at mach 0.86 max. Glad you are enjoying the 777…

…Good day. Hope this helps

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#17

If I may ask, what is the max air speed at FL280 and above?

#18

Flight time in most airlines is calculated in Ground speed.

#19

Mach 0.87…It varies in knots dueto the high altitude

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#20

Thanks for the help :) Have a great day :)

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#21

Glad it was worth the help. Feel free to ask when any problem occurs…besides this is a community :)

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