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Take Off From High Density Altitude Aerodrome
mnorris created the topic: Take Off From High Density Altitude Aerodrome
Long time reader, first time contributor!
I recently undertook an exam where there was a question about taking off from an aerodrome with a high density altitude. The question was about settings for the mixture control and the throttle for take off.
I understand that the mixture isn't at full rich for high density altitude take offs, but I can't seem to find anywhere about throttle setting. One of the options was for full power, and others were for not quite full power.
I've searched the Internet for some help, and come up with about three different answers.
Can anyone offer any suggestions?
bobtait replied the topic: Take Off From High Density Altitude Aerodrome
This is a technique used only in exceptional circumstances such as a very high strip and very hot and humid day e.g. Nu Guinea highlands in Summer. The mixture is not really being leaned, it is being restored to it's normal fully rich setting because the density altitude has produced a super-rich condition. The engine would be running roughly at full power in these conditions.
The mixture is leaned just sufficiently to restore smooth running. It would be normal to use full throttle since the mixture has been restored to the appropriate condition. Remember that a fully open throttle does not necessarily mean 100% power is being delivered. Under very high density altitude conditions, full throttle may only be producing about 70% power because the manifold pressure is lower, so there is no reason why you cannot have the throttle wide open. I know of some crop-duster mates who have used this technique with full load, high strip and hot day.
Note that when you use a take-off chart to establish a take-off weight, the chart assumes that the engine is developing its expected power. That will not be available with a super-rich mixture so the chart may over estimate the take-off weight permitted.
However, remember that this is not really a normal situation and in a country like Australia, it would probably rarely be necessary.