Your equation isn't quite correct.
It should be
TA = IA + (4 x IA/1000 x ISA Dev) + (27 x (QNH – Subscale))
Where
TA = true altitude,
IA = indicated altitude,
Subscale = altimeter subscale setting,
ISA Dev = ISA temperature deviation
Strictly speaking the term "Pressure Altitude" means the altimeter reading when 1013 is set on the subscale. So if we were to use Pressure Altitude instead of indicated altitude in the above equation we would be applying the temperature error from the 1013 level upwards.
My use of TA in ,y previous post was an error. Using the above equation we can solve the problem as follows.
TA = IA + (4 x IA/1000 x ISA Dev) + (27 x (QNH – Subscale))
ISA Deviation = -15
True Altitude = 15000 ft
QNH = 1003
TA – (27 x (QNH-Subscale setting) = IA + ( (4 x IA x ISA Dev)/1000 )
Inserting the data provided in the question gives
15000 – ( 27 x (1003 – 1013) = IA + (4 x IA x (–15) ) / 1000 )
15000 + 270 = IA – 0.06 IA
15270 = 0.94 IA
15270 / 0.94 = IA
16244 = IA
So Indicated Altitude = 16244 feet
This is not quite the correct answer of 16230, but it is pretty close to it.
Last edited by keith williams; 19th Feb 2012 at 16:25.