Jun 8, 2009

High Altitude Airspeed Envelope

A friend recently asked me an intresting question, in light of the speculations surrounding the Air France flight 447 disaster:

What would be the stall speed versus the "normal" cruising speed on a plane like this?

At middling altitudes ... like 28,000 there's a ton of spread. Let's say 120 knots stall speed (no flaps) - cruise at 450 knots. But at high altitude cruise it gets a little more complicated because of the speed of sound and incipient shock waves.

As an aircraft climbs above approx. 28,000 feet, the maximum speed the wing can fly gets slower (shock waves start to form). At the same time the lowest speed it can fly at gets faster (thinner air, complicated as well by shock waves).

This phenomenon contributed to some crashes of the original 707 and DC8 jet transport airplanes as the full effects of turbulence and maneuvering on this tightening envelope weren't fully appreciated. At such high altitudes any increase in G-loading, causes these two speed extremes to come even closer together.

The point where they meet, as displayed on a performance chart, has been nicknamed "coffin corner" because of this history.

(Typical Buffet Boundary Chart)

If an A330 is cruising at his maximum altitude for his weight, there could be as little as 10 to 20 knots room to spare on either side of the "envelope". Which is why when we encounter "significant" turbulence or mountain waves, most pilots prefer a lower altitude.

Here is a typical presentation of the speed envelope on a modern "glass" panel. The airspeed tape on the left shows the allowance between high and low speed buffet - about plus/minus 15 knots in this example:


david said...

Could you clarify what you're showing in the last illustration? I had believed that stall speed in IAS or CAS would be the same at sea level or FL380, and that only the TAS differed.

Aluwings said...

David, Essentially that's correct up to the point where high speed compressibility begins to be factored in. At some point the wing can be close to the stall angle of attack and also close enough to the speed where shock waves form (critical mach number for that wing) - so that the interference of the shock waves cause airflow separation prior to the "normal" stall angle.

Here's a better description of Mach Buffet Boundary.

In my posting the ASI shows 'cautionary' indictions above and below the current airspeed. Depending on the aircraft, these warnings have a little bit of maneuvering room built in so the aircraft could safely bank up to 15 degrees for a gentle turn without suddenly being thrown into the critical ranges. But moderate turbulence or bank angles beyond 15 degrees swiftly reduce the margins to zero ... Time to descend.

Anonymous said...

.86? Holy moly, a corporate jet (lear, gulfstream)?? Don't think the bus can do that! :)

Aluwings said...

I don't know which aircraft that is, but I looked up the B747 because I think it is the fasted cruiser of all the jet transports. If you notice the planform of the 747 you notice right away that it has a highly-swept wing. Maybe because it was designed in the days when jet fuel was still cheap? Anyway here's what I found for speeds:

Cruising speed at 35,000 feet
Mach 0.855
(570 mph, 495 kn, 917 km/h)

Maximum speed at 35,000 ft
Mach 0.92
(614 mph, 533 kn, 988 km/h)