Why is overspeed at 350 kn but at a cruising altitude like 38k ft. the overspeed is 270 kn. and why does the speed indicator switch from knots to mach. can someone explain this, thanks!
This is because ground speed is different than airspeed.
Most airliners cruise at Mach 0.84-85 so:) and if you go above 0.94 i believe you can get speed violations so
That happens in all aircraft, there is less air up at high altitude, so it’s easier to overspeed because there’s less drag. And ground speed is different than airspeed, and when you get that high, 100kts airspeed could be 800kts, whoch gives you violations. which is why it’s over speeding.
Come to the community!
At high altitudes Air is less dense and the drag acting upon an aircraft depends on true airspeed (TAS) since indicated airspeed is (IAS)=(1/2)pV^2. High speed flight increases parasitic drag and at transonic speeds drag starts to increase exponentially. Mach number is the TAS compared to the local speed of sound, which IAS to Mach number lowers the higher you go.
Refer to this for all the information you could want, specifically Pg. 5-45 for what you’re asking. Feel free to ask further questions.
Ground speed is the measure of how fast you are traveling over the ground, airspeed is how fast air molecules are being rammed into what’s called a pitot tube. Naturally, since the air is thinner higher up, it takes less airspeed to go the same ground speed as at a low altitude.
There is a Mach limit and a 350 kts limit, so whichever is lower results in overspeed.
I assume that’s part of the reason why about fl280 mach speed is used, you you don’t accidentally overspeed when step climbing.
However for most (or every?) the mach limit takes over before fl280 so don’t set 350 knots in climb, set 290-310 (above 10000 obviously).
Most airliners never go at Mach 0.84-85 so. It depends on model and usually;
737s cruise at 0.77
320s cruise at 0.78
330s cruise at 0.80
777s cruise at 0.82
747s cruise at 0.84
787s and 350s cruise at 0.85
Adding on to that, as the plane get higher it will enter a area call Q Corner or Coffin corner. This is when the plane will stall if a couple knot slower and will over speed couple knot faster at the same speed. For an example, if your KIAS shown 350kt at high altitudes, you will go overspend at 355kt and stall at 345kt.
Because speed is measured by a difference of pression. More you go high more the pressure is weak more you have to go fast to compensate.
In high the speed is a real airspeed in Mach but you keep on going higher the pression goes weaker but you don’t accelerate to compensate.
It’s complicated to explain but never reach stall indicated airspeed.
Higher, hotter, faster
That’s how it goes,
Regarding to speeds your aircraft is limited by the loads on the structures and the formation of shock waves which result in buffet.
Vmo is the max speed limited by the loads
Vmmo is the max. Mach number , the speed that prevents you from reaching the critical Mach number where the air at some points of the wing begins traveling at the speed of sound.
As you climb, you will pass the crossover altitude where you switch from IAS to Mach, typically around FL280. At lower altitudes your aircraft is limited by IAS because the speed of sound is higher, at higher altitudes the speed of sound is lower so the aircraft is limited by Mach number.
Supersonic flow is the limititation of your airliners MMO. The true airspeed where you reach MMO decreases the colder the air gets, means the higher you fly. That is until reaching the tropopause around FL350 where the temp. Stops decreasing and the TAS remains more or less constant.
The higher you fly, the lower is your MMO and the higher is your stall speed. That’s why you see the “ barber pole “ indication in your cockpit, the upper part indicating your Mmo and the lower your stall speed.