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  1. #1
    Peter Dowson
    Guest

    Question for airliner pilots

    I'm relaying this question from Ray Proudfoot and I here because I believe we
    have some airliner pilots, do we not?

    Ray and I had a lengthy discussion about when to change from STD to QNH
    pressure during the descent. Ray thought it should be when or close to the TL,
    disregarding current clearances, but I said I thought it should be when we
    get our first clearance to an altitude (rather than a Flight Level), and are
    setting this altitude on the MCP.

    My logic here is that the autopilot is targetting on whatever the altimeter
    is set to. In particular, the descent rate to make a specific clearance will
    be calculated and the autopilot will proceed on the basis that the
    altimeter will read the target altitude when it gets there -- but it could be up
    to 1000 feet difference depending on the QNH/STD difference. If the target
    altitude is the first valid one below the TL then that could be quite a big
    change.

    To me, it seems much more logical to allow the automatic mechanisms to do
    their job correctly and smoothly by providing them the correct inputs they
    need.

    Of course the same logic applies to the climb to a Flight Level, but of
    course in this case the check would be that you were on STD pressure.

    Comments from any real-world pilots would be appreciated. Which is "right"?
    Are there rules? References? If the "right" way is the one I feel is
    illogical, why is it right?

    Bear in mind that I am using an EFIC control panel and PM's lovely PFD/ND
    which allows be to swap between QNH and STD with ease, a single button
    press.

    Regards,

    Pete


  2. #2
    Matt Olieman
    Guest

    Re: Question for airliner pilots

    18,000' MSL and above..... always at set at 29.92 and 17,999' and below,
    local ALT settings. As far as I know there are no variances to that rule.

    Matt O.

    "Peter Dowson" wrote in message
    news:411493.82130@wb.onvix.com...
    > I'm relaying this question from Ray Proudfoot and I here because I believe


    > we
    > have some airliner pilots, do we not?
    >
    > Ray and I had a lengthy discussion about when to change from STD to QNH
    > pressure during the descent. Ray thought it should be when or close to the


    > TL,
    > disregarding current clearances, but I said I thought it should be when

    we
    > get our first clearance to an altitude (rather than a Flight Level), and
    > are
    > setting this altitude on the MCP.
    >
    > My logic here is that the autopilot is targetting on whatever the
    > altimeter
    > is set to. In particular, the descent rate to make a specific clearance
    > will
    > be calculated and the autopilot will proceed on the basis that the
    > altimeter will read the target altitude when it gets there -- but it could


    > be up
    > to 1000 feet difference depending on the QNH/STD difference. If the

    target
    > altitude is the first valid one below the TL then that could be quite a
    > big
    > change.
    >
    > To me, it seems much more logical to allow the automatic mechanisms to do
    > their job correctly and smoothly by providing them the correct inputs

    they
    > need.
    >
    > Of course the same logic applies to the climb to a Flight Level, but of
    > course in this case the check would be that you were on STD pressure.
    >
    > Comments from any real-world pilots would be appreciated. Which is
    > "right"?
    > Are there rules? References? If the "right" way is the one I feel is
    > illogical, why is it right?
    >
    > Bear in mind that I am using an EFIC control panel and PM's lovely PFD/ND
    > which allows be to swap between QNH and STD with ease, a single button
    > press.
    >
    > Regards,
    >
    > Pete
    >



  3. #3
    Peter Dowson
    Guest

    Re: Question for airliner pilots

    On 4/21/2006 6:26:48 PM, Matt Olieman wrote:
    >18,000' MSL and above.....
    >always at set at 29.92 and
    >17,999' and below,
    >local ALT settings. As far as
    >I know there are no variances
    >to that rule.


    Ah, only in America. Sorry, I don't fly there. Most parts of the world have
    things called Transition Altitudes and Transition Levels, which are
    distinct, and vary quite considerably (and which are mostly much lower than the US
    18,000).

    Anyway, thank you, but I have my answer now, from another source, quoted from
    the UK AIP:

    =====================================
    The extracts from the UK AIP below explain in more detail (I've highlighted
    the most important bits). In simple terms, though, when outbound you set QNH
    until ATC clears you to climb to a FL at which point you set 1013.2/29.92
    unless a vacating report is required at an intermediate altitude. When
    inbound and cleared from a flight level to an altitude you set the QNH as soon
    as you have vacated the FL unless you are asked for a vacating report at an
    intermediate FL in which case, you keep 1013.2 until you've passed that
    level.

    Here are a few examples.

    1. You're flying at 4000ft on the QNH 1023 when you receive clearance to
    FL80. As soon as you leave 4000ft, you re-set your altimeter to 1013.2.

    2. You're flying at 4000ft on the QNH1023 when you receive clearance to FL80
    to report passing 5000ft. In this case, you leave 1023 set until you've
    passed 5000ft and then you re-set your altimeter sub-scale to 1013.2.

    3. You're flying at FL120 on 1013.2 when you receive clearance to descend to
    6000ft, QNH 1021. As soon as you vacate FL120, you re-set your altimeter
    sub-scale to 1021.

    4. You're flying at FL120 on 1013.2 when you receive clearance to descend to
    6000ft, QNH 1021 to report passing FL80. In this case, you leave 1013.2 set
    until you have passed FL80 in the descent and then you re-set your
    altimeter sub-scale to 1021.

    ==============================

    Handily, all this is exactly what I've been doing for years, simply because
    it seemed so logical!

    Regards,

    Pete


  4. #4
    David Rabiner
    Guest

    Re: Question for airliner pilots

    Pete,

    I thought Mode C transponders send altitude based upon what the altimeter is

    reading. If that's the case, then a prematurely changed altimeter setting
    would send erroneous altitudes to ATC, which would, of course, impact ATC's

    ability to separate traffic and accurately inform pilots of the location of

    nearby aircraft.

    Warm regards,
    David


  5. #5
    David Rabiner
    Guest

    Re: Question for airliner pilots

    Well, let me correct myself. Mode C transponders apparently send pressure
    altitude. ATC's equipment then adjusts the pressure altitude transmission
    to the current altimeter setting. As a private pilot, I guess I should have

    known this.

    Again, this is also all USA stuff, with the steering wheel on the left.

    David


  6. #6
    Peter Nielsen
    Guest

    Re: Question for airliner pilots

    When an aircraft has to climb until or above the Transition Altitude,
    Flightlevels are used, and the altimeter is set to the standard pressure 1013
    (QNE). If the aircraft stays below the Transition Altitude it is called
    Altitude.
    When an aircraft has to descend below the Transition Level, Altitudes are
    used, and the altimeter is set to the local QNH. If the aircraft has to level
    off above the Transition Level, you should use Flight Levels.

    For the United States, the Transition Altitude is 18000ft. Above this
    Altitude, Flight Levels are used. For Belgium, the Transition Altitude is fixed at
    4500 ft. For Germany Transition Altitude is 5000 ft, and for the
    Netherlands Transition Altitude is 3000 ft. The transition Layer is always minimum
    1000 ft thick. So if the local pressure is 1013 Hpa, the Transition Level is
    FL040 (Netherlands). One (1) Hpa is about 30 ft. So if the local pressure
    is 1000 Hpa, the Transition Layer will get smaller, so the Transition Level
    will be higher. How much? Well at least 13 x 30 = 390 ft. The nearest
    Flightlevel is then FL045 as Transition Level


  7. #7
    Peter Dowson
    Guest

    Re: Question for airliner pilots

    On 4/22/2006 3:37:04 PM, Peter Nielsen wrote:
    >When an aircraft


    Your tutorial is very nice, and thank you for it, but my question was way
    beyond such elementary matters, which I do really understand thoroughly I
    assure you. As it is I did receive answers from another source which I included
    earlier in this thread.

    Thnk you anyway for your contribution.

    Regards,

    Pete



  8. #8
    Don McKellow
    Guest

    Re: Question for airliner pilots

    Real life info:

    On climb, nver set STD untill actually at Transition Altitude.
    Except in USA it is accepted to set QNH on descent when cleared below
    Transition Level, however for accurate TCAS operation, on descent the QNH should
    be set at exactly Transition Level.

    Regards
    Don


  9. #9
    Peter Dowson
    Guest

    Re: Question for airliner pilots

    On 4/23/2006 7:19:30 AM, Don McKellow wrote:
    >On climb, nver set STD untill
    >actually at Transition
    >Altitude.


    Actually the UK AIP seems to disagree, and I've now found references in CAP
    85, Aviation Law for PPL applicants, Appendix 11, which states "after
    clearance to climb above TA has been given and climb commenced, vertical position
    will, unless spcially requested by ATC, be expressed as a flight level
    provided that the aircraft is no more than 2000 feet below TA".

    Interesting that the 2000' part doesn't seem to appear in the AIP reference.
    The CAP 85 I have is dated Feb 1997 so things may have been changed since
    then. I don't have any of the new European joint authority books I'm
    afraid.

    Regards

    Pete


  10. #10
    Peter Dowson
    Guest

    Re: Question for airliner pilots

    On 4/23/2006 7:19:30 AM, Don McKellow wrote:
    >however for accurate
    >TCAS operation, on descent the
    >QNH should be set at exactly
    >Transition Level.


    Are you sure about TCAS operation being dependent upon the pilot setting the
    altimeter correctly? I really hope that is not the case. Surely TCAS works
    on pressure differences, as for Transponder -- in fact doesn't it rely on
    the transponder settings, which use pressure altitude, not some possible
    misreading off the altimeter dial?

    Anyway, I have found a website from which I can get the actual rules. It has
    to be subscribed to, but that is free, so I will try to join it and come
    back with the actual rules.

    Regards,

    Pete


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