In the photo the IAS is 104 kts. but the Machmeter is reading .704 Mach. Now, given that at 40,000 ft.Mach 1 would be approx. 575 kts., at 70,000 ft wouldn't Mach .704 be right around 450 kts? Or am I justmissing something?

Indicated vs true airspeed?

Hello!I have a question related to your screen shot, and perhaps someone could clarify it for me....In the photo the IAS is 104 kts. but the Machmeter is reading .704 Mach. Now, given that at 40,000 ft.Mach 1 would be approx. 575 kts., at 70,000 ft wouldn't Mach .704 be right around 450 kts? Or am I justmissing something?You have to admit, though... quite a view out the canopy, huh? :( Good luck with your flights, squawkvfr!Kampai,Alan :(

Alan,The reason is the difference between IAS (Indicated Air Speed) shown by your Airspeed Indicator and TAS (True Air Speed). (If wind is zero than TAS = GS (Ground Speed)).As you climb and ambient air pressure decreases with altitude, IAS will also dedcrease (both static and ram air pressures decrease), while TAS is not affected by altitude.Now stall speed is tied to IAS, while Mach number, and the aircraft's MMO (Mach Maximum Operating Number), are tied to TAS (after a slight adjustment for temperature). So as you climb higher you have to fly faster to stay above stall speed (as your IAS decreases), but you are limited by MMO in terms of TAS, so eventually you find yourself in a corner, in the Coffin Corner: Just a few knots slower and you stall as your wing's angle of attack exceeds the critical angle of attack, and just a few knots faster and you also stall as airflow is detached from the top surface of the wing due to supersonic shockwave formation:Bear in mind the more into the Coffin Corner you fly, the more exposed you are to your own errors (or the Auto-Throttle's) in maintaining proper airspeed, and to changes in load factor as you turn or hit turbulence.Cheers,- jahman.

What do you mean conceptual error? Isn't that a real altitude (FL600)?...

Exactly, it's not a "real" altitude!Rarely will the 29.92" (QNE)setting of you altimeter coincide with Kollsman Window pressure setting of a second altimeter placed on the ground directly below your aircraft and reading proper elevation above sea level (QNH).Your altimeter always reads "pressure altitudes" relative to the setting of the altimeter's Kollsman Window. When that setting happens to be 29.92" (by convention), the pressure altitudes measured (in hundreds of feet" are called Flight Levels.But altitude, its not.Cheers,- jahman.

Jahman,Thanks so much for the explanation; your descriptions and diagrams made this much easier to understand! :( squawkvfr -- just remember... when you're in Coffin Corner, please don't poke yourself in the eye with the straw ifyou have to scratch your nose! :( Kampai,Alan :(

No, in the context of this thread 29.92" is simply a pressure datum, i.e. the reading of the altimeter's Kollsman Window that the altimeter must be set to.When the altimeter's Kollsman window is set to 29.92", then the altimeter altitude in hundreds of feet are Flight Levels, but only for altitudes at or above the Transition Level (the lowest Flight Level, FL180 in the U.S. but as low as FL30 in other countries).And now, time for a FL600 anecdote:In a famous SR-71 story...Los Angeles Center reported receiving a request for clearance to FL600. The incredulous controller, with some disdain in his voice, asked, "By the way, how do you plan to get up to 60,000 feet?" <-- note the conceptual error!The pilot (obviously a sled driver), responded, "We don't plan to go up to it, we plan to go down to it."He was cleared...Cheers,- jahman.

Touche

FL is a measurement unit devoid of any legal or regulatory content in and of itself, so the FL will be whatever your aircraft altitude is, rounded to the nearest 100 ft, provided your altimeter is set to standard pressure 29.92".Cheers,- jahman.

IIn the USA CFR 14 § 1.1 - General definitions reads:"Flight level means a level of constant atmospheric pressure related to a reference datum of 29.92 inches of mercury ..." CFR 14 § 91.179 - IFR cruising altitude or flight level reads: "(a) In controlled airspace. Each person operating an aircraft under IFR in level cruising flight in controlled airspace shall maintain the altitude or flight level assigned that aircraft by ATC. "

There can be such a thing as FL100, just not in the US. FL are used above the transition altitude, where the altimeter has to be set to the standard 29.92InHg or 101.3KPa, which is 18000 feet in the US, but in Europe it can vary greatly, being as low as five to sixthousand feet.

Infact, in the UK the standard transition altitude outside controlled airspace is 3000ft, and so in the UK, the lowest flight level is FL35.

IIn the USA CFR 14 § 1.1 - General definitions reads:"Flight level means a level of constant atmospheric pressure related to a reference datum of 29.92 inches of mercury ..." CFR 14 § 91.179 - IFR cruising altitude or flight level reads: "(a) In controlled airspace. Each person operating an aircraft under IFR in level cruising flight in controlled airspace shall maintain the altitude or flight level assigned that aircraft by ATC."

Exactly.Cheers,- jahman.

Exactly.Cheers,- jahman.

Then when did you it's devoid of any legal or regulatory content when it clearly isn't? "Bust" an assigned FL and you'll hear about it!

Then when did you it's devoid of any legal or regulatory content when it clearly isn't? "Bust" an assigned FL and you'll hear about it!

You need to separate the measurement provided by an instrument from the meaning assigned to specific values that measurement might take.Let's try with a terrestrial car analogy: "50 MPH", as displayed by your car's speedometer, is a speed devoid of any legal meaning. "Speed limit: 50 MPH" means go over 50 MPH and you're busted. In fact you can have the latter without the former, as in: Driver: "But my speedometer is broken/missing!" Officer: "That's your problem, you still broke the speed limit according to my radar gun/the trail of flaming sagebrush left behind your car, etc.".Same with Flight Levels: FL180 on your altimeter (with the Kollsman Window setting of 29.92") is a measurement devoid of legal meaning, because it's just a measurement. But go over FL180 when flying VFR in the U.S. and you're busted.Cheers,- jahman.

Flight level is legally defined in the FARs - in the UK it's legally defined by statute. That's why it's based on 29.92 inches of mercury in the US and 1013.2 hectopascals in the UK. Without such definitions it would be meaningless.

Infact, in the UK the standard transition altitude outside controlled airspace is 3000ft, and so in the UK, the lowest flight level is FL35.

In the UK Transition Level is the lowest Flight Level available for use above the transition altitude. With a transition altitude of 3000 ft, It can range from FL25 to FL50 depending on QNH. See Section 5.2 and Appendix A of the Manual of Air traffic Services Part 1http://www.caa.co.uk...CAP493Part1.pdf

Flight level is legally defined in the FARs - in the UK it's legally defined by statute. That's why it's based on 29.92 inches of mercury in the US and 1013.2 hectopascals in the UK. Without such definitions it would be meaningless.

It doesn't matter who defines flight levels. One thing is the flight level itself and the other is any legal restriction associated with a particular flight level.Proof 1: If the preceding were not so, then flight levels would not exist in unregulated airspace (airspace devoid of legal rules). But FLs do exist in unregulated airspace.Proof 2: You can measure the height of a mountain in FLs as is done for MSAs, yet no FAA FARs apply to mountains (yet!).Proof 3: If any legality was associated to the measurement of FLs, then setting your altimeter to any other setting than 29.92" would be illegal. Yet no such illegality occurs. What's illegal is the actual flying of your aircraft at an unassigned FL, regardless of what your altimeter says.Cheers,- jahman.

It doesn't matter who defines flight levels. One thing is the flight level itself and the other is any legal restriction associated with a particular flight level.Proof 1: If the preceding were not so, then flight levels would not exist in unregulated airspace (airspace devoid of legal rules). But FLs do exist in unregulated airspace.Proof 2: You can measure the height of a mountain in FLs as is done for MSAs, yet no FAA FARs apply to mountains (yet!).Proof 3: If any legality was associated to the measurement of FLs, then setting your altimeter to any other setting than 29.92" would be illegal. Yet no such illegality occurs. What's illegal is the actual flying of your aircraft at an unassigned FL, regardless of what your altimeter says.Cheers,- jahman.

You fail to understand that without a definition, the term flight level means nothing. In order to have a meaning it is legally defined and therefore has a legal meaning.Unless it was legally defined the FAR that reads "...shall maintain the altitude or flight level assigned that aircraft by ATC." would be enenforcable.

You fail to understand that without a definition, the term flight level means nothing. In order to have a meaning it is legally defined and therefore has a legal meaning.Unless it was legally defined the FAR that reads "...shall maintain the altitude or flight level assigned that aircraft by ATC." would be enenforcable.

"Feet", for example, are not defined in the FARs. So according to you we would have the very strange situation that FLs are "legal" wheres distances from clouds are not.Legality means actions either are legal or are illegal. Measurements in and of themselves cannot be legal or illegal. They just are. And it doesn't matter where the unit used in the measurement is defined.In the case of FLs, being defined in the FARs does not make FLs "legal".But if you fly at the wrong FL your action will be illegal. OK?Cheers,- jahman.