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When an aircraft's max speeds are stated in a manual or an information sheet ie. Grumman Goose max speed 175kts cruise speed 166kts, it's in KIAS right? Probably a silly question I know, but I'm wondering if it's generally referring to indicated airspeed or ground speed.
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yes, KIAS.

(Knots Indicated Air Speed)


I like to think of KIAS as 'pressure on the front window'.

(it isn't, it is a speed that comes from pressure measured in a small forward facing tube on the outside of the aircraft. But theat is the 'pressure from forward movement' and it is similar to the pressure on the windshield. I think that makes it easyer.)


If you exceed the max speed, the pressure on the front of the plane, but also on the wings, and tail, on every forward facing bit, just gets too high. And at too high a speed the plane just tears apat. It stucturally fails. Lika a kite on a string in a fierce gust of wind.


at low altitude air is dense. Many particles per liter.

At high altitude it's much less dense. Less particles per liter.

Tht means to get the same pressure on the windshield you have to fly faster. (faster in relation to speed over ground.)


If 2 flights, one at low altitude and one at high altitude your KIAS is the same (pressure on windshield same0

Then at high altitude the actual speed over ground is much higher.



You can also think of KIAS as "speed in relation to the air around me"



Your KIAS is the most important speed to know I think.

Like you said, the Max speed is in KIAS.

But also, the ma flaps speeds.

Those are also structural speeds. If you fly faster then the Max speed for flaps 20, and you have flaps 20 extended, then yourflaps will get damaged.

In the real world they get bent and get stuck, or would even fall off the aircraft completely. If that happens pilots will have to do a partial flaps up landing.

same with gear extension speed.

And maybe most importantly, stall speed is in KIAS.

The airflow overr the wings is what provides lift to prevent a stall, and that depends on KIAS. Not on speed over the ground.

It that depends on speed and air density. It depends on pressure of the airflow.



My physics is a bit rusty, but I think that's the gist of it.

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Thanks for that. Excellent explanation. I'm surprised that safety has taken priority over marketing in this respect, in that, if I was a dirty salesman I'd try to sell the aircraft on the basis of it's ground speed (at max KIAS at it's highest altitude for maintaining it). That way no one would come back and ask for their money back when they've flown from A-B in a 100kt headwind and say "I was punching her out at max speed and the bloody thing took longer to get to Aunty Mary's than my Subaru!".


I dunno.

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if I was a dirty salesman I'd try to sell the aircraft on the basis of it's ground speed (at max KIAS at it's highest altitude for maintaining it).


You are expressing a mistake I see on sim sites all the time, thinking that groundspeed means something in terms of aircraft performance. Ground speed is true airspeed corrected for wind under current conditions. And ground speed is meaningless, except for determining how long the current flight will take, as calculated for this moment. As the wind changes, groundspeed changes, even though the aircraft may be in a steady state (constant speed) in relation to the air mass that it is in.


True airspeed is much more meaningful, in the sense that it is repeatable, and it is faster than indicated in most cases. Yet a given TAS only occurs under specific conditions of temperature, air pressure (density altitude) and power setting, as well as whether (and how much) the aircraft is climbing or descending, but TAS in level flight at max continuous power at the optimum altitude under standard conditions is the only possible "best" speed that could be advertised (and often is).


Note that all advertised figures (max speed, cruise speed, range, etc.) are figured in no wind conditions. All pilots are aware of wind effects, so a "dirty salesman" wouldn't get far with that kind of approach, so your scenario is all but impossible.


Also note that salesmen aren't the ones who specify the speeds in the manual. That's done by testing, to FAA specs, under FAA regulations. Safety is, indeed, a prime purpose of the flight manual and/or operator's manual for each aircraft.


As to your initial post, max speeds in performance charts (speed vs altitude vs power setting vs fuel consumption, etc.) in the manual are true airspeed. Indicated (actually calibrated, or CAS) airspeed is used for the various "V" speeds, such as Vne (never exceed), Vso (stall), Vle (landing gear extended), etc. since they affect aircraft behavior, as opposed to time to get somewhere.


Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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