Altitude Questions
Thread Starter
Joined: Sep 2003
Posts: 12
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From: Some Back Alley Street
Altitude Questions
Any scientists out there that can figure out the answers to these two questions?
1. Given an indicated altitude of 10,000 feet and an actual OAT of -20C, you set your altimeter to a local station setting of 29.62". If the station elevation is 2500 feet, what is your actual altitude?
A.9250
B.9550
C.9850
2. Given a field elevation of 1500 feet, altimeter 1000 millibars, calculate pressure altitude.
A.1110 feet
B.1890 feet
C.890 feet
I know that the answers to both are (B) but can't for the life of me figure out the methodology used to get there.
1 inch HG = 34 Millibars
1 inch HG = 1000 feet
Appreciate any help. Thanks.
1. Given an indicated altitude of 10,000 feet and an actual OAT of -20C, you set your altimeter to a local station setting of 29.62". If the station elevation is 2500 feet, what is your actual altitude?
A.9250
B.9550
C.9850
2. Given a field elevation of 1500 feet, altimeter 1000 millibars, calculate pressure altitude.
A.1110 feet
B.1890 feet
C.890 feet
I know that the answers to both are (B) but can't for the life of me figure out the methodology used to get there.
1 inch HG = 34 Millibars
1 inch HG = 1000 feet
Appreciate any help. Thanks.
Joined: Apr 2002
Posts: 150
Likes: 0
From: Bohol, Philippines
The formula you need is
TA = IA + (IA - FE)*ISADEV/°K)
TA=True Altitude
IA=Indicated Altitude = 10,000
FE=Field elevation=2,500
ISADEV=ISA deviation=-15
°K=Temperature in °C at IA +273=253
Inserting your figures in the formula gives
TA=10,000 + (7500*(-15)/253) = 9555
So answer B is correct
TA = IA + (IA - FE)*ISADEV/°K)
TA=True Altitude
IA=Indicated Altitude = 10,000
FE=Field elevation=2,500
ISADEV=ISA deviation=-15
°K=Temperature in °C at IA +273=253
Inserting your figures in the formula gives
TA=10,000 + (7500*(-15)/253) = 9555
So answer B is correct
Grandpa Aerotart
Joined: Jun 2000
Posts: 4,579
Likes: 3
From: SWP
Or an easier to remember formula for question 1
+/- 4' per degree of ISA Devn x (ht/1000)
Where ht = the height above the temperature datum, which is the airfield.
+4x15(7.5)
So at an indicated altitude of 10000' your altimter will be overeading by 450' giving a true altitude of 9550'.
Q2 is simply at a matter of 'winding' your altimeter up to 1013 to get pressure alt. 13Hp x 30' per Hp = +390' hence B.
Chuckles.
PS but I do like the TA formula with degrees Kelvin, being a naturally enquisitive bloke so I'll write it down and attempt to memorise it
+/- 4' per degree of ISA Devn x (ht/1000)
Where ht = the height above the temperature datum, which is the airfield.
+4x15(7.5)
So at an indicated altitude of 10000' your altimter will be overeading by 450' giving a true altitude of 9550'.
Q2 is simply at a matter of 'winding' your altimeter up to 1013 to get pressure alt. 13Hp x 30' per Hp = +390' hence B.
Chuckles.
PS but I do like the TA formula with degrees Kelvin, being a naturally enquisitive bloke so I'll write it down and attempt to memorise it
