Indicated vs actual altitude

[weirdjim]

Is this a certified airplane? If so you still have your steam altimiter? What is the altitude difference between the two?

If I hear this again, I am going to (digitally) scream. THEY ARE ALL CERTIFIED AIRPLANES. Some standard, some utility, some aerobatic, and some experimental. STOP USING THE WORD CERTIFICATED as though it was holy.

Jim

 

[John Collins]

Can I ask a dumb question (and therefore learn something!). I thought it worked the following way:

1. Pilot periodically adjusts the Kollsman offset based on nearby ATIS/AWOS reports. In theory his altimeter reads altitude above sea level with not too much error. This error is known when the pitot static check is done and the tech checks the altimeter against various (pressure) simulated altitudes.

In theory the altimeter reads altitude above mean sea level. But the theory is based on a standard atmosphere and standard temperature lapse rate. The temperature error introduces very significant errors. At the surface where the baro altimeter setting is determined, there should be "not too much error". However, the higher you are above that point, temperature can introduce more and more error, on the order of hundreds or even thousands of feet depending on the temperature and altitude. That is one reason why an extra 1000 feet is used in mountainous areas for obstacle clearance. If you don't want to hit the mountain top on a very cold day, either stay extra high or use a GPS to determine your actual altitude, as it has much less error than a barometric altimeter and is unaffected by temperature.