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Old 23rd May 2019, 00:18
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plotplot
 
Join Date: Apr 2019
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Nav True alt calcs

Hi xCartzy,

Can you confirm that the true alt calcs always use AGL even when the question has given infromation using area QNH instead of local? Because you get wildly different answers depending on which number you use. Here is an example from Higgins:

You are cruising at A070 directly above a mountain with elev 6420ft. QNH is 1001 OAT -14degrees. The RADALT would indicate:

using the formula above; 4ft x (580 / 1000) x 15 = 35 (7000-35= 6965 = 545 RADALT)

However if you substitute AMSL for AGL; 4 x (7000/1000) x 15 = 420 (7000-420= 6580 = 160ft RADALT)

Using a CR3 if you line up A070 with -14degrees in the true altitude window, you can see that true altitude is in fact around the 6600 mark or just under, making the second formula correct.

Can you please clarify if I am missing something. Cheers
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