Low overspeed speed

[Blawrence 0]

I am flying the 737-800 with autopilot at 36,000 feet with autothrottle. However, the speed indicator shows overspeed just above 250 kias.Is there a way to fix this?

 

Tyler Lawrence

[bjratchf 80]

Max indicated airspeed limits reduce with increasing altitude. Try Mach .80 as a number to work with during cruise, don't go much over that. We use mach speed above FL270 anyway. 

 

This is perfectly realistic and normal.

[g_precentralis 191]

OMG, that's so unrealistic, real 738 can fly much faster than 250kts!!!! 

[hopskip 253]

Air pressure gets thinner the higher you go. Pitot probes air pressure and compare it to static probes to derive speed. If the pitot is blocked like what happened to Air France 447 the airspeed reads 0kts even in flight. If the static port is blocked the airspeed can read much higher than it really is. See if you can figure out why indicated airspeed is so much lower than the Ground Speed readout on the nav display!

[Spin737 102]

OMG, that's so unrealistic, real 738 can fly much faster than 250kts!!!!

kias vs. ktas

[AirborneZed 168]

Your looking at airspeed going into the pitot tube. You're actually going 400+ kts across the ground. The atmosphere is like the ocean. The higher above the surface, the less pressure.

[davierosoft 13]

If you read the manual you will find how true realistic it is.

The 737 VMO limit is 340/.80 mach wichever comes first.

The barber pull indicator will remain to 340KTS until around 26000 fts, then it will start to decrease as it is not more linked to Kias but Mach number.

Under fl260 you will have 340kts limit (wich is less than .80 mach), above you will have .80 mach limit wich is less than 340kts.

The reasons were already posted, air pressure varies with altitude, and the aircraft main indicators show CAS (calibrated air speed) aircraft velocity in the sorrounding air.

[Spin737 102]

.82M

[davierosoft 13]

:blush: Yes, sorry, my mistake

[mrtk311 34]

Try a cruise speed of .785

[hopskip 253]

Check the second screen from the left. In the top left corner of that screen (the "Navigation Display Screen") you will see some numbers. One of these is "GS"
GS = "Ground speed" This is how fast you are going. If you are going 470 knots GS, then in 1 hour you will have travelled 470 nautical miles. You are going 470 nautical miles per hour! Normal people call this 470 knots.

 

 

If you slow down to 300 nautical miles per hour at 38,000ft high, you will stall, because the air molecules going over the wing are not enough to produce lift. It's almost like you are going 140 knots with the flaps up! STALL!
why are you stalling at 300 knots?

well you indicated speed shows 140 knots, your stick shaker is going off, there's some voice saying "Stall! Stall!" and you are falling to the ground like a brick.

 

Now you know why they have indicated speed indicators.

[davierosoft 13]

Ground speed is the orizontal true speed of the plane versus ground, however on a plane is the most useless info we can have. It is used just to plan or info.

Ground speed cannot reflect stall speed, you can theorically have a 0 ground speed with 200kts of air speed but 200kts of winds.