Given the following information, what is the approximate reading expected on the altimeter, when the pilot changes the subscale setting during a climb through the transition altitude?
Transition altitude: 18 000 ftTransition level: FL175
QNH: 1035 hPa
Assume 30 ft per 1 hPa
When climbing through the transition altitude, the pilot will change the altimeter subscale from QNH to standard pressure setting (1013.2 hPa) so they can then fly pressure altitudes (Flight Levels) above that altitude. In this case, the QNH pressure is higher than standard, so the indicated altitude is higher than pressure altitude. We are therefore expecting the altimeter reading to decrease during this subscale setting procedure. We can calculate this decrease using a barometric correction.
Pressure Difference = 1035 - 1013 = 22 hPa
Altitude difference = 22 hPa x 30 ft/hPa = 660 ft
Pressure Altitude = 18 000 ft - 660 ft = 17 340 ft
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