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This question already has an answer here:

I'd like to know the interval of altitudes airliners usually cruise at (and its reasons)? And if there are international restrictions about this?

Also I read somewhere that recently about a restriction that airliners should not fly bellow 26000 ft over egypt. How hard that restriction would be?

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marked as duplicate by fooot, digitgopher, mins, aeroalias, Ralph J Nov 8 '15 at 2:17

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I think there are already plenty of answers here to give you a clue:

Normally, atitude restrictions give a top limit. A minimum altitude is hard to enforce when one engine fails - then the aircraft can only continue at a lower altitude (typically around 15,000 ft for a two-engined jet) or the cabin pressurization fails (now the aircraft needs to continue below 10,000 ft).

This is speculation, but maybe the 26,000 ft limit is caused by information that rebels in the West Sinai have gotten hold of shoulder-launched anti-aircraft missiles. Those have a fairly low upper limit to their effectiveness.

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  • $\begingroup$ thank you and what would be typical top limit? $\endgroup$ – Azad Nov 7 '15 at 17:57
  • $\begingroup$ There is no "typical" top limit, and ATC stops at 50,000 ft, well above regular airliner capabilities. You have en-route top limits normally for individual airplanes in case of system failure (where they are flightworthy up to that altitude) or to keep departing and arriving traffic apart near an airport. Also, some restricted airspace (say, for military exercises) can be overflown only above a certain altitude. $\endgroup$ – Peter Kämpf Nov 7 '15 at 18:27
  • $\begingroup$ @PeterKämpf the ceiling for ATC isn't constant. over the US, class G starts at 60,000 ft. $\endgroup$ – casey Nov 8 '15 at 2:33
  • $\begingroup$ @casey above FL600 is class E. $\endgroup$ – Mike Brass Feb 12 '18 at 6:11
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It depends on the type of airliner. They all fly high to take advantage of a higher "true airspeed" (TAS).

Smaller turboprops like the Beech 1900, SA226, and SA227 might cruise in the low to mid 20,000 foot band. Jets will usually cruise above 30,000 feet.

The air is thinner at high altitudes which reduces drag on the airplane and allows it to slip through the air at a higher "true" airspeed. Turbine engines are optimized to run at high altitudes and by running in to 20,000 - 30,000 foot altitude band, they can still generate enough power to propel the aircraft along at a couple hundred knots using far less fuel than they would at sea level.

The airspeed indicator in a jet cockpit will indicate something like 250 knots, the true airspeed will actually be 400 knots, and the ground speed will equal true airspeed plus or minus the wind component. Why the big difference between indicated airspeed and true airspeed? The thin air "tricks" the airspeed indicator! There are simply fewer air molecules at 20,000 than at seal level and since the airspeed indicator in the cockpit needs air molecules to determine the speed, the thinner air fools the indicator into thinking the plane is going slower than it really is.

The true airspeed corrects for this by taking into account the thinner air and temperature to tell the pilot "hey, your airspeed indicator tells you that you're speed is 250 knots. But since you're up high and the air is thinner, I've done the calculation to determine your REAL speed through the air...it's 400 knots!"

So...

At sea level, a jet will indicate 250 knots. The atmosphere is thick at sea level so the indicated and true airspeed will be fairly close.

The same jet at 30,000 feet will indicate 250 knots but the true airspeed will be much higher.

Taken to an extreme, a spacecraft in orbit has a speed of many thousands of miles per hour. What's the airspeed? ZERO! There is no more atmosphere so the airspeed indicator (which relies on air molecules, remember) can't register an airspeed. The ground speed of the spacecraft is extremely high, though. Now, toss in a thin atmosphere and your indicator will begin to show you a speed. It will be low because the air is thin but your actual (TRUE) airspeed will be very high.

As far as the Egyptian altitude restrictions go, I haven't a clue.

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