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6 Answers

True Altitude

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General Aviation

I live in mountainous terrain.  So CFIT is not what I would like to have in my logbook.  I notice that not one nav-log has this to be calculated.  Why?  Is it not important?  If I do calculate my true altitude should I then make the change inflight?  Example  I am at 5000 feet IA.  Altimeter is set correctly to the local setting, my True Altitude calculation says I am 4,750.  Should I climb to 5250 IA to achieve 5,000?  If I didn't, does Radar see me at 4750 while my altimeter reads 5000?

Thanks

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6 Answers



  1. Wes Beard on Feb 24, 2014

    Nathan,

    Some interesting questions. First are you talking about flying VFR or IFR? If you are on an IFR flightplan the MEA’s in mountainous terrain have been raised to counter tue effects of pressure and temperature deviations on an altimeter.

    If you are flying VFR and using the MEFs for each quadrangle a CFIT accident is very possible. They only guarantee you 300-400 ft separation from the mountain peaks and only 100 ft if an antenna is there.

    Every airplane with a transponder has a blind encoder that reports the pressure altitude you are currently flying. ATC will convert those values for the altimeter setting for the area you’re currently. If you are flying at 5200 they will know but on a VFR flight it isn’t that important.

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  2. Nathan Gerhards on Feb 25, 2014

    Wes,
    Thank you. I do know that you can get your pressure altitude in many ways one many may not really know. Use your XPDR, cycle through the page.
    So flying IFR, the controller asks me to be at 5000, should I climb to 5250? If not so basically they want me to be physically at 4750 but they ask me to fly to 5000.

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  3. John D Collins on Feb 25, 2014

    You fly an MSL altitude, what your altimeter reads when the proper barometric pressure is set into the Kollsman window. MSL altitude is not corrected for temperature, so there can be a significant difference between your true altitude and the MSL indicated altitude. This error increases with the distance from and height above where the altimeter setting is determined. That is why in mountainous areas, IFR minimum cruise altitudes require a 2000 foot terrain clearance whereas in non mountainous areas it is only 1000 MSL. On an approach, you set the local airport altimeter setting and by virtue of flying the approach, you are getting closer to the altimeter setting location and altitude, so the error gets smaller on approach and goes away completely by the time you are on the ground.

    The FAA is in the process of providing pilots guidance on corrections at certain cold weather airports, but I don’t believe it has been published as of yet.

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  4. Wes Beard on Feb 25, 2014

    If you are in the United States, you fly the altitude cleared by ATC and your true altitude will be lower in cold weather and higher in warm weather. We aren’t really worried about true altitude under IFR because the errors have already been corrected but under VFR you are wise to make those corrections but since you can see the mountain coming you should be able to avoid it pretty easily.

    Our friends in Canada are much more worried about this than we are. It makes sense because they can get much colder than we do in the U.S. They are required to make cold weather error calculations and fly those other altitudes just like you suggest.

    The chart the canadians use is in the FAA’s AIM handbook. Read section 7-2-3

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  5. SleepNpilot on Feb 26, 2014

    I flew quite a bit in Southeast Alaska. I understand completely about mountain flying and cold weather flying. I am a low time IFR pilot now and so your answers were perfect, both people who responded. Thank you very much. “true altitude” seems to be an area of training that is put has not being necessary to care about. Now an IFR in the mountains I would think its that much important. I feel this concept should be drilled upon more than lets say True Airspeed. You can’t hit the air and make it hurt.

    Thanks again

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  6. Mark Kolber on Feb 26, 2014

    So flying IFR, the controller asks me to be at 5000, should I climb to 5250?

    Nathan. Think about it for a minute. You are in flight in solid IMC. ATC instructs you to climb and maintain 5000 feet. Let’s throw in a bit of moderate turbulence since, after all, you are in bad weather in mountainous terain.

    The controller wants you to:

    (a) Look at the dial in front of you and fly 5000′ indicated.

    (b) read the OAT, reset the Kollsman to 29.92 to get the pressure altitude, whip out your E6B, line up the 5000′ on the inner scale with the temperature, read the true altitude on the scale and then get around to climbing.

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