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Altimeter adjustment

Discussion in 'Student Pilot and Further Learning' started by petetheprinta, Jun 20, 2011.

  1. petetheprinta

    petetheprinta Active Member

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    Hi All, I am after an opinion on the following.: (hopefully it makes sense) forgive me if the jargon is wrong :doh: I am interested only in theory. I am not taking into account mapreading, ATIS, cold fronts, warm fronts, local QNH, area QNH or any other variables, it is purely a difference in altitude of airfields and indicated AGL V's actual AGL. NOTHING ELSE MATTERS FOR THIS PURELY HYPOTHETICAL QUESTION

    I take off from airfield (A) which I know is at an elevation of 0' (sea level). and have set altimeter accordingly to read 0'
    I fly to airfield (B) which I know is at an elevation of 1000' ASL

    Hypothetically, on arrival at (B) if I join circuit at 1000' as indicated by my altimeter I am 1000 AMSL but only 0' AGL (obviously wrong), I therefore have to think about it, do some math and climb to 2000' indicated to give me my circuit height (1000'AGL) OR:
    Do I somewhere along the flight path readjust altimeter by subtracting 1000' (Difference between A+B) by turning knob, to accommodate known altitude of airfield (B) therebye joining circuit at 1000' AGL and 1000' indicated on alt (2000' ASL in fact) without having to do math at landing stage, relieving my workload .
  2. brilin_air

    brilin_air Aircraftpilot Groupie

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    Hi Pete,
    Well done on the new plane.
    You don't have to adjust the altimeter until the QNH changes, as it automatically changes the height reading on the main scale the higher or lower that you fly. If it didn't do that your altimeter would always read the height that you have set it at, at the start.

    Brian
  3. petetheprinta

    petetheprinta Active Member

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    Hi Brian, thanks for that, I have rewritten my IP in the hope it makes more sense.
  4. skeptic36

    skeptic36 Active Member

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    Yep that's what you do except that you don't need to add to your workload because if you have a look in the ersa before you start you can do the calculation and make a note of it in your flight plan.

    Regards Bill
  5. jakej

    jakej Member

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    You should beable to get the PF QNH via ATIS from there and dial that into the Altimeter then compare Alt reading to aerodrome elevation. IF there is a significant (more than 100') difference then the Alt is out of calibration or, as you probably have the 'Chinese' Altimeter, it is U/S.

    Jake J
  6. rocketdriver

    rocketdriver Student Mentor

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    Adding to what others have said, you will presumably have consulted a map and marked your proposed track upon it and noted the height of the high ground along and on either side of your proposed flight. On your flight plan, you could then annotate your desired cruising height at each waypoint so as to maintain a safe and legal altitude from this waypoint to the next. For your destination airport, you would note either the cct height or the airport height, whichever you wish. Then all you do is make sure the altimeter reads the appropriate height as you travel along your path. If you are close to a weather system like a cold front remember to make allowance for the likelihood of a changed QNH close to the weather system ..... rising pressure makes the altimeter read low ..... This happens, for instance, if you take off shortly after the passage of a cold front and particularly if you then travel West.
    As an aside, and hoping to clarify what is happening when you set the airfield height on the altimeter, you are actually calibrating the instrument to account for today’s atmospheric pressure ...once that’s done, if the instrument and its installation are serviceable, it will correctly read your height above sea level until and unless the sea level air pressure changes ....
    Easy Peasy! :cheezy grin:

    Cheers

    RD
  7. fly_tornado

    fly_tornado Well-Known Member

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    Interesting, you need to know the alt of the airstrips you are using the alt will do the rest.
  8. Wayne T Mathews

    Wayne T Mathews Well-Known Member

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    Well said David.

    Wayne.
  9. petetheprinta

    petetheprinta Active Member

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    Thanks David, I have rewritten question in the hope of better explaining myself. o' and 1000' is an easy one to do the maths and fly at 2000' indicated, it seems to me that say 397' and 1634' take a little more figuring.... Imagine landing, on top of everything else I am trying to work out by adding or subtracting, circuit height, height at final turn, over the fence etc. on top of actually flying the plane. Wouldn't it be easier to adjust alt to suit (so every landing no matter what the AGL is, is same indicated). Forget the maths and concentrate on the flying. (think of this from a learners point of view)
  10. petetheprinta

    petetheprinta Active Member

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    Thanks David point taken.
  11. rocketdriver

    rocketdriver Student Mentor

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    Hi Pete
    Another thing you will find is that, with a standard circuit ht of, say, 1,000 agl, you will find that, after a few circuits, your eyeballs will calibrate to what the view of the runway should be at each major point in the circuit. So the trick really is to know what ht the runway is (1,000 ft in your sample), and start your circuit at that height plus 1,000 indicated on the altimeter. Then everything will fall into place. You won't have time and nor should you be looking at the altimeter on short final . .... you'll be looking at your aiming point and then as you flare, transferring your gaze to the far end of the runway .... but your instructor will take you through all of this as you progress through your training ....
    cheers
    RD
  12. petetheprinta

    petetheprinta Active Member

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    Hi RD, believe it or not I have my licence, (albeit recently) I just always question why we do things the way we do, my instructor hated me LOL.
  13. Crezzi

    Crezzi Well-Known Member

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    This is entirely valid & standard practice in some countries - when inbound to your destination, the altimeter is adjusted down by the height of the airfield so it will read 1000' in the circuit and 0 on the ground.. If flying locally you simply set 0' before takeoff. Its called QFE but isn't used in Aus possibly because at many airfields there isn't enough adjustment in the altimeter.

    Cheers
    John
  14. biggles

    biggles biggles

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    Pete ,
    I can see where you are getting confused and essentially the concise reply by brillin_air is correct . You will find that with (a)the duration of your flights and (b) the conditions you will be flying in , there will be little change in the atmospheric pressure between your point of departure and your destination , hence the altimeter will read accurately at your destination . The important thing is to ensure that the correct elevation is entered at your point of departure and , providing this is done , your altimeter will read correctly at your destination also - no need to remember the elevation at your departure point or worry about arithmetic .

    eg If I leave Bairnsdale ( Elev 160'AMSL ) for Mt Hotham ( Elev 4260'AMSL ) a distance of approx. 100 n.m., and providing I have set the Altimeter to 160' before departure , it will be reading near enough to correct at Mt Hotham , and I will join circuit at 5260' as indicated on my Altimeter.
  15. davidh10

    davidh10 Guest

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    Always good to question things. It usually results in a better understanding.

    The correct answer has already been given, so I won't repeat it, but consider if you were to reset your altimeter to read AGL at the destination as you proposed, instead of AMSL. Then how do you:-
    • Indicate your altitude at the 10 mile inbound call.
    • Indicate your altitude overhead, as if you are flying to an airfield where the operational runway is unknown, you would overfly to look at the windsock and that has to be above the highest circuit height (remember 1500' for high performance a/c).
    • Respond to another aircraft's call to identify your position, which has to be 3D. ie. distance, direction from AD and altitude. These requests can and do occur at any random time.
    Perhaps this helps explain further, why is is done the way it is.

    By the way, your examples of having to calculate circuit height for airfields having altitudes that don't correspond with an integer 100'..., such as 397' and 1634', use rounding to the nearest 100', so AD=397' ==>Circuit=1400'; AD=1634' ==> Circuit= 2600'.

    As a new pilot, if you can hold altitude within +/- 20' you are doing pretty well :-)
  16. petetheprinta

    petetheprinta Active Member

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    David, My ignorance is showing again, what is " AD" Thanks
  17. davidh10

    davidh10 Guest

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    Rather than leave this as a rule of thumb, which is only sometimes correct, it would be better not to assume and check the weather forecast before leaving. That will give you Area QNH and any subdivisions within the Area. If your destination is in another Area, then you need to get weather for there too. Thus, you know before you leave what the QNH is at your point of departure and if it changes along the track, where the changes are effective and the QNH values. You can then adjust you altimeter during the flight as you enter an area of different QNH, so that it will compensate for the change in pressure in that locale. Listen to Area frequency for changed weather announcements and if you aren't sure, as others have said, ask Airservices for the QNH at <location>.

    The way you ask is:
    1. Call: Melbourne Centre, <aircraft type><registration>, Request QNH for <location>.
    2. Response: <aircraft type><registration>, QNH for <location>, 1017.
    3. Readback: QNH 1017, <registration>.
  18. davidh10

    davidh10 Guest

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    AD = Aerodrome.

    No problem.
  19. davidh10

    davidh10 Guest

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    Hey David; I work in IT and until I became involved with aviation, I thought IT was bad for acronyms... too many TLAs (three letter acronyms). :roflmao:
  20. petetheprinta

    petetheprinta Active Member

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    Of course it is!! :frusty:

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