Why do I lose airspeed when climbing very high?

Q:
Hey guys, recently i did a flight on a 777 and the ft plan called for me to climb and maintain FL 390 as i got past about FL 320 i began to see a slow but constant decrease in my Air Speed. I was holding full throtles and by the time i got to FL 390 i was down to 180 KIAS, but 300 - 400 Ground Speed. There was apoint i was loosing speed yet my ground speed was constant.

Why ? and way to avoid that ?

tanveer


A:
Hi Tanveer,

The reason is because as you get higher the air thins. As this is the case the air can no longer support the weight of the aircraft as well, so you should expect to see the climb angle decrease. If you were looking at the climb profile of an aircraft, you should expect to see similar to the following:



Notice that at high altitudes the plane takes a lot longer to climb 1000ft.

However in your case, you are trying to force the aircraft to maintain a particular Vertical Climb Rate e.g. 1800fpm which on the chart above would look like a straight line. This is leading to the struggling plane trying to help you by trying to convert its airspeed into more climbing power, in an effort to help maintain the impossible climb rate.

Therefore the correct way to climb is actually to forget vertical speed altogether and base your climb on airspeed. If you are unsure how to do this, please ask below and someone else can talk you through it.

TornadoWilkes


The airspeed indicator is nothing but a pressure gauge, calibrated in units of speed. Higher altitudes and higher temperatures mean thinner air, thus less pressure into the pitot tube at the same actual speed through the air. The system is calibrated to read correctly at sea level (0 feet MSL) at a pressure of 29.92 inches of mercury (Hg) and 5? F (1? C). Under any other conditions, the IAS will not be accurate in relation to your actual speed (there are ways to calculate the corrections), but will be great for noting things such as stall speed and climb speeds, since the wings, props, etc. are affected exactly the same as the ram air pressure into the pitot tube.

So what you are seeing is the difference between Indicated AirSpeed (IAS) and True AirSpeed (TAS). And to confuse the issue still further, groundspeed is TAS corrected for winds. For instance if you are doing 300 kts TAS and have a 50 kt headwind, your groundspeed is 250 kts. Conversely, with a 50 kt tailwind your groundspeed would be 350 kts. Only with zero wind would TAS and groundspeed match.

Every aircraft has a best rate of climb IAS which gradually decreases with altitude due, among other things, to reduced engine power as you climb (yes, even in jets).

Jet pilots will transition to using the mach number for airspeed as they climb, typically somewhere in the mid-20 flight levels, depending on aircraft and conditions, as it becomes a limiting factor for the airframe as you get higher -- yet another complication.

Larry N.

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