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QNH for aircraft.

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  • Gabriel
    replied
    Originally posted by Evan View Post

    No, in lieu of, meaning before vox radio, or when it wasn't availalble. Q codes were also sent via signal flags but probably not out of airplanes.
    I know. I was not contradicting you, just showing the contrast. I never used the Q codes in lieu of vox radio, I've only used them in voice radio communications (not only in aviation).

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  • Evan
    replied
    Originally posted by Gabriel View Post

    In the times of vox radio, you just say "what's the QNH?", "QNH 2099".
    No, in lieu of, meaning before vox radio, or when it wasn't availalble. Q codes were also sent via signal flags but probably not out of airplanes.

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  • Gabriel
    replied
    Originally posted by Evan View Post

    Originally, used in lieu of vox radio, they could function either as a query or a reply, depending on the context.

    So QSL could mean "do you acknowledge?" or "acknowledged". QNH could mean "what is the current sea-level pressure?" or "this is the sea-level pressure".
    In the times of vox radio, you just say "what's the QNH?", "QNH 2099".

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  • Evan
    replied
    Originally posted by Gabriel View Post

    What does roger mean and why? What does PAN mean and why? What does SOS mean and why?

    The Q-codes are very old, there are hundreds of them, they were developed for radiotelegraph (i.e. Morse signals) and they were NOT created as a mnemonic or an acronym.

    Q-codes that I have actually used:
    QNH (current sea-level pressure)
    QNE (standard sea-level pressure)
    QFE (current ground level pressure)
    QDM (heading to station, used in ADF navigation)
    QDR (heading from station, used in ADF navigation and equivalent to radial)
    QTR (current time)
    QSL (message acknowledged, equivalent to roger)
    Originally, used in lieu of vox radio, they could function either as a query or a reply, depending on the context.

    So QSL could mean "do you acknowledge?" or "acknowledged". QNH could mean "what is the current sea-level pressure?" or "this is the sea-level pressure".

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  • Gabriel
    replied
    Originally posted by 3WE View Post

    No flame war but, bull crap. ATC in the USA used to call it the altimeter setting. The weather man called it the barometric pressure, adjusted to sea level. QNH REFLECTS NONE of that.

    Knots DOES REFLECT Nautical miles per hour...nauts...knots...

    STD REFLECTS standard or sexually transmitted diseases

    QNH is Quincy North High school.

    What does roger mean and why? What does PAN mean and why? What does SOS mean and why?

    The Q-codes are very old, there are hundreds of them, they were developed for radiotelegraph (i.e. Morse signals) and they were NOT created as a mnemonic or an acronym.

    Q-codes that I have actually used:
    QNH (current sea-level pressure)
    QNE (standard sea-level pressure)
    QFE (current ground level pressure)
    QDM (heading to station, used in ADF navigation)
    QDR (heading from station, used in ADF navigation and equivalent to radial)
    QTR (current time)
    QSL (message acknowledged, equivalent to roger)

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  • 3WE
    replied
    Originally posted by Evan View Post
    QNH literally means "What should I set on the subscale of my altimeter so that the instrument would indicate its elevation if my aircraft were on the ground at your station?"

    But of course, now it just means the parameter itself. Like knots.

    But what about STD? Now there's an acronym with a history...
    No flame war but, bull crap. ATC in the USA used to call it the altimeter setting. The weather man called it the barometric pressure, adjusted to sea level. QNH REFLECTS NONE of that.

    Knots DOES REFLECT Nautical miles per hour...nauts...knots...

    STD REFLECTS standard or sexually transmitted diseases

    QNH is Quincy North High school.


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  • Evan
    replied
    Originally posted by 3WE View Post
    WT Phugoid does Q stand for as well as N, H, F, and E??? What's wrong with plain English (and gray areas)?
    Tradition. Why do we call airspeed in knots when nobody is casting a knotted line out the window? Why isn't a rudder called a 'yawder'. It isn't there for steering a course. You do that with the... yoke... but there are no oxen involved.

    Q comes from the early wireless Q codes whereby ships at sea would send queries via morse code. Each three-letter code began with Q (which I assume stood for 'query' but this is debatable).

    QNH literally means "What should I set on the subscale of my altimeter so that the instrument would indicate its elevation if my aircraft were on the ground at your station?"

    But of course, now it just means the parameter itself. Like knots.

    But what about STD? Now there's an acronym with a history...

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  • 3WE
    replied
    This is a bad mix of jargon and slang. The words “zero” are so rarely used. The acronyms AGL and MSL were not used- these at least stand for something intuitive. And it’s used more to avoid obstructions than to know when you will touch down.

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  • Gabriel
    replied
    Ok, I understood that you said that with QNH the altimeter would show zero when on the runway. Sorry for misunderstanding you.

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  • ikeharel
    replied
    Originally posted by Gabriel View Post

    That's wrong. As 3WE and myself explained, QNH is setting the altimeter to show zero at mean sea level and hence to show the real altitude above sea level. If you set your altimeter to QNH and land at an airport that has an elevation of 4500 ft, your altimeter will show 4500 ft at touchdown, not zero.

    If you are in an airplane at the ramp and want to know the QNH, a good way is just to adjust the atmospheric pressure setting so as the altimeter reads the airport's elevation. Then the pressure that you have set is the QNH. Another good practice is to get the QNH from the ATIS or the tower and compare that it matches the setting you just got with the previous procedure, or just set the QNH you got from the tower or ATIS and then check that the resulting altimeter reading matches the airport elevation.

    What you said is not called QNH but QFE.
    Right, all I tried to explain that knowing the QNH helps the pilots to determine what would be the zero-elevation on any runway.
    Sorry not to be more precise.
    Sincerely,
    Ike

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  • Gabriel
    replied
    Originally posted by ikeharel View Post
    The simple explanation is this:
    Setting the QNH on the altimeter for a specific airport, runway or any airstrip, meaning that this would be the "zero" height for landing (or take-off).
    For example, if Denver airport is about 1 mile elevation from sea-level, setting the QNH reading for that specific place enable the pilots to count real altitude on landing to the zero real elevation.
    Regards,
    Ike
    That's wrong. As 3WE and myself explained, QNH is setting the altimeter to show zero at mean sea level and hence to show the real altitude above sea level. If you set your altimeter to QNH and land at an airport that has an elevation of 4500 ft, your altimeter will show 4500 ft at touchdown, not zero.

    If you are in an airplane at the ramp and want to know the QNH, a good way is just to adjust the atmospheric pressure setting so as the altimeter reads the airport's elevation. Then the pressure that you have set is the QNH. Another good practice is to get the QNH from the ATIS or the tower and compare that it matches the setting you just got with the previous procedure, or just set the QNH you got from the tower or ATIS and then check that the resulting altimeter reading matches the airport elevation.

    What you said is not called QNH but QFE.

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  • ikeharel
    replied
    The simple explanation is this:
    Setting the QNH on the altimeter for a specific airport, runway or any airstrip, meaning that this would be the "zero" height for landing (or take-off).
    For example, if Denver airport is about 1 mile elevation from sea-level, setting the QNH reading for that specific place enable the pilots to count real altitude on landing to the zero real elevation.
    Regards,
    Ike

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  • Gabriel
    replied
    3WE, I like your answer better, it is more clear than mine.

    The only thingie is that the reason why all planes switch to the standard 29.92 at 18000ft all over the world, except outside North America where 3000 ft above the ground ft is typically used as transition altitude, s that it's good to have all the planes, even ones coming from different sectors, using the same altimeter setting so the altimeters are consistent among them, and also so the pilot doesn't need to re-adjust the altimeter setting during cruise flight.

    Regarding QNH, QNE and QFE.
    QNH: Altimeter setting for altitude above mean sea level. Used for "low level" flight (below the transition altitude)
    QNE: Altimeter setting for standard atmospheric pressure art sea level, i.e. 29.92 inches of mercury, 760 mm of mercury, or 1013.3 hecto pascal or mili bar, used for "high level" flight (above the transition level).
    QFE: Altimeter setting for altitude above the station (so the altimeter indicates zero when on the runway regardless of the runway elevation).

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  • 3WE
    replied
    I read Gabriel's answer...I didn't like it.

    To some extent, an altimeter IS a barometer that measures the absolute pressure...as you go up, pressure goes down.

    However, the ambient air pressure varies too...that means your altimeter will go up and down, even though the plane is parked on the ground.

    There is an adjustment on the altimeter (Gabriel said this) where you ENTER the current sea-level pressure- and this adjusts the altimeter to still be accurate to the actual altitude even though the pressure has changed. If you set the altimeter on 29.92 (average) and the actual weather service sea-level barometric pressure is 30.20, the altimeter will INDICATE you below the ground level. 29.60 inches and the altimeter INDICATES you above the ground….

    Somewhere around 18,000 feet, all folks are supposed to set their altimeters to 29.92" or 1013 mb…The smart people believe this is a good idea since airpressure can deviate from average with height-.

    As Gabriel said, you want things to be accurate and not fly into mountains and the ground and stuff.

    PS- one of the reason I hate Gabriel's reply is that it's full of acronym garbage...what's wrong with "Barometric Pressure" or "Altimeter Setting"? WT Phugoid does Q stand for as well as N, H, F, and E??? What's wrong with plain English (and gray areas)?

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  • Gabriel
    replied
    The QNH is the local sea-level air pressure. So if the station is not at sea level but at an elevation, it is the pressure that you would have if you dug a hole until reaching sea level and measuring the pressure at the bottom of the hole. In other words, if you are at an elevation of say 2250 ft, you get an altimeter, adjust the pressure until it reads 2250 ft, and the pressure you read in the pressure window of the altimeter will be the QNH.

    When the altimeter is set at QNH, the altitude shown in the altimeter is the real altitude above sea level (ok, there might be some minor errors, but that is the idea).
    In this way, if you want to land at a runway that has an elevation of 3200 ft, you know that if the altimeter shows 4200 ft you are 1000 ft above the runway. Or if there is a mountain with a 8000 ft peek, if your altimeter shows 9000 ft you know you will clear it by 1000 ft. You would not know any of that if your altimeter is set at other than QNH. Finally, other airplanes flying in the vicinity will have the same QNH set in the altimeter, so if one plane is at 4000 ft indicated and the other at 5000 ft indicated, they are 1000 ft apart vertically. That is still true if both planes have the same altimeter setting even if it not QNH, but if one s QNH and the other one has QNE or QFE, then when both planes are flying next to each other at the same real altitude, the altimeters would display different altitudes.

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