altitude calculations

nimtz

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Never came across questions like this before (or its been way too long) and haven't a clue how to figure them out as relates to actual altitude. Any pointers or attempts to replace the blacked out lightbulb in my head would be appreciated.

"Given an indicated altitude of 10,000 ft, OAT -20C, and altimter setting 29.62. If the station elevation is 2500 ft what is your actual altitude?"

Answer: 9550 ft
My density altitude calculation comes out to 8500 ft so not sure what else I'm missing.

"At a temp of ISA -25C how much altitude should be added to the MEA."

Answer: 10%
Is there a rule of thumb I missed out on here?
:confused:
 

Singlecoil

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nimtz said:
Never came across questions like this before (or its been way too long) and haven't a clue how to figure them out as relates to actual altitude. Any pointers or attempts to replace the blacked out lightbulb in my head would be appreciated.

"Given an indicated altitude of 10,000 ft, OAT -20C, and altimter setting 29.62. If the station elevation is 2500 ft what is your actual altitude?"

Answer: 9550 ft
My density altitude calculation comes out to 8500 ft so not sure what else I'm missing.
:confused:
I'll tackle the first one:

Here is a quick formula found on google. I came up with 9555 ft using it on your problem. Density altitude is not what you are trying to solve here, you are looking for true altitude, or indicated (calibrated) altitude corrected for non-standard temperature. Remember, unless the air you are flying through is at standard temperature, your altimeter is lying to you. This is a particular problem when operating in mountainous terrain on a cold day. As this problem indicates, you thought you would have cleared that 9500 ft mountain when your altimeter indicated 10,000 ft, but you smacked right into it. This applies to all aircraft, from C-152 to B-747. Air data computers DO NOT correct for this phenomena, but enhanced ground proximity warning systems do. So in the bigger jets, your altimeter is still lying to you, but the ground prox will bark at you because it applies the conversion.

Relationship of true and calibrated (indicated) altitude:

TA= CA + (CA-FE)*(ISADEV)/(273+OAT) where

TA= True Altitude above sea-level
FE= Field Elevation of station providing the altimeter setting
CA= Calibrated altitude= Altitude indicated by altimeter when set to the altimeter setting, corrected for calibration error.
ISADEV= Average deviation from standard temperature from standard in the air column between the station and the aircraft (in C)
OAT= Outside air temperature (at altitude)
 

nimtz

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Singlecoil said:
I'll tackle the first one:

Here is a quick formula found on google. I came up with 9555 ft using it on your problem. Density altitude is not what you are trying to solve here, you are looking for true altitude, or indicated (calibrated) altitude corrected for non-standard temperature. Remember, unless the air you are flying through is at standard temperature, your altimeter is lying to you. This is a particular problem when operating in mountainous terrain on a cold day. As this problem indicates, you thought you would have cleared that 9500 ft mountain when your altimeter indicated 10,000 ft, but you smacked right into it. This applies to all aircraft, from C-152 to B-747. Air data computers DO NOT correct for this phenomena, but enhanced ground proximity warning systems do. So in the bigger jets, your altimeter is still lying to you, but the ground prox will bark at you because it applies the conversion.

Relationship of true and calibrated (indicated) altitude:

TA= CA + (CA-FE)*(ISADEV)/(273+OAT) where

TA= True Altitude above sea-level
FE= Field Elevation of station providing the altimeter setting
CA= Calibrated altitude= Altitude indicated by altimeter when set to the altimeter setting, corrected for calibration error.
ISADEV= Average deviation from standard temperature from standard in the air column between the station and the aircraft (in C)
OAT= Outside air temperature (at altitude)
Thanks man. Just realized what I was looking for after the post, but would of taken sometime to figure how to crunch the numbers. Haven't tackled this kind of stuff for a long time.
 

Stifler's Mom

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Man, these Instrument Checkride Orals are getting harder and harder.
 
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