Posts: 9
Joined: Wed Jul 19, 2000 4:47 pm

Skin Temp During Cruise

Thu May 17, 2001 7:44 pm

Knows someone how hot or cold gets the outer skin/paint of an typical airliner like A320 or B737 during cruise altitude (e.g Fl390) and a normal airspeed. I knows that for example the concorde gets hot, especially in the nose section (to 135°C!) but I`ve got no idea how it is on an airliner, wich flies not that fast and high.Note the influence of friction and speed/oat. Any ideas and/or facts?
Posts: 78
Joined: Wed Nov 29, 2000 1:54 am

RE: Skin Temp During Cruise

Thu May 17, 2001 10:25 pm

The Ram Rise on a typical jet aircraft at cruise altitude & speed is about 25°c (it varies with a change in Mach number).

If the standard atmospheric temperature is -55°c at 35,000', the Ram Air Temperature would be about -30°c.

Most jet aircraft use a RAT (Ram Air Temperature) Gauge in the cockpit to determine when to use Anti-ice equipment. The RAT Gauge tells you the approximate temperature of the skin of the leading edges of the aircraft.

Hope this helps,
Posts: 106
Joined: Mon Apr 30, 2001 10:10 pm

RE: Skin Temp During Cruise

Fri May 18, 2001 6:15 am

I saw a recent article in the NY Times regarding new UA747-400 service between JFK and Hong Kong that flies over the north pole. It mentioned that the flight crew would monitor the fuel temp, and if it got too low they we fly faster to warm up the wings -- is this true? Can you control external temperature to this extent?

Posts: 78
Joined: Wed Nov 29, 2000 1:54 am

RE: Skin Temp During Cruise

Fri May 18, 2001 11:13 pm

Freezing fuel is a concern in all aircraft that fly long-range flights. In the long-range planes I am familiar with, fuel is heated by a Fuel-Oil Heat Exchanger, and then some of that warm fuel goes to the engine and the rest goes back to the fuel tank to heat the fuel in the tank.

A Fuel-Oil Heat exchanger is a method of heating cold fuel and cooling hot oil. The Cold Fuel lines running next to the oil lines cause the oil to cool down. At the same time, the Hot Oil lines cause the fuel to heat up. Hence, at heat exchanger.

Increasing speed will not significantly increase the skin temperature to warm the fuel. It will only cause the aircraft to burn more fuel, and possibly run out before it gets to where it is going!

Now, for the subject of flying over the North Pole.

This is gonna sound kinda strange but it is true, consider a flight at FL390 (39,000'). Where would the temperature be Colder, over the Equator or over the North Pole?

The answer is, it would be colder over the Equator.

Temperature decreases with altitude up to the Tropopause. The Tropopause is where the Troposhpere meets the Stratosphere. Once an aircraft reaches the Tropopause, the temperature starts to remain constant or increase! The troposhere varies from around 60,000' at the equator to about 25,000' over the poles. This means that the Temperature is continuously decreasing up to 60,000' at the equator, and up to 25,000' over the poles. Above these altitudes, the temperture no longer decreases.

Example: Compare the Temperatures aloft over Alaska to the Southeast US at 39,000'.

See this link for winds and temps aloft:
See below to learn how to decode the numbers.

The temps over Alaska vary from -43 to -53°c.
The temps over the S.E. US vary from -55 to -57°c.

While these locations are not very close to the poles or the equator, they do illustrate the fact that the temperatures aloft are warmer near the poles than near the equator.

If you look at these forcasts in the link above, you will also notice that in Alaska, the temperatures remain fairly constant or increase between 30,000 and 39,000'. In the Southeast, they continuously decrease all the way up to 39,000'.

*** Decode Winds/Temps Aloft ***
If you don't know how to read the winds and temps aloft, this is how.

This is the number group for a given altitude: 297055
Break up the numbers in 2 digit groups: 29 70 55
29 = Wind Direction (290° West-Northwest)
70 = Wind Speed (70 Knots)
55 = Temperature (-55°c)

All temperatures above 24,000' are negative, therefore, you will not see + or - in front of the temperature.

Hope this helps,

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