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Incorrect Altitude

Pro Member Trainee
whittke1 Trainee

When I select a plane and the airport, my altitude is already at 400 sometimes. How can I reset this to 0?

Pro Member Chief Captain
Karlw Chief Captain

two things

1. The airport you may be starting at is above sea level.

2. The barometric pressure may be lower therefore causing the alt to indicate higher.

At any airport you will hear the atis or awos say altimeter 2992 or 3019 etc this stands for inches of mercury. If you look for the barometer in your airplane you must set it to the awos or atis info. However 18,000ft + all aircraft set there alt to 29.92

Good luck.

P.S welcome to flyaway Cheers!

Pro Member Trainee
whittke1 Trainee

Ok I understand that so that means that when I'm landing my altitude will be off because I started the flight wrong. I hope I'm explaining this correctly, but I'll never be able to reach the correct cruising altitude because I'll either be too high or too low

Pro Member Chief Captain
Karlw Chief Captain

Lets say you are flying a 737 IFR from EGLL to LIRF. on a standard day your barometer would read 29.92 but a standard day is also 25C or 59F. However an area of low pressure or high pressure can move into your area of dep or arrival. Thus causing an inaccurate readout on alt if the correct barometric pressure is not dialed in. Back to the 737 upon reaching 18,000ft you must dial in 29.92 because Most IFR traffic is above 18,000ft therefore it makes more sense for atc to keep everyone with the same alt readout as to avoid collisions. Now upon descending threw 17,999ft you must tune is the nearest field with an atis or awos or the airport you are landing at if you can. And listen for "Altimiter 30.01" and whatever it say's weather it be 30.01 or 10.01 etc you must dial in that number or you are going to have some very upset passengers lol. it's good to keep updated on ATIS at the end of every ATIS transmission you will hear "You have information "XRAY" If you listen to atis from the same airport 15min after XRAY you will hear you have information "ZULU" or any alphabetical term. When you hear a different inform code that means atis has been updated.

Anyways I'm off to bed. If you have any question post them and I will get back to you asap.

Cheers Very Happy

Pro Member First Officer
Bob (Traches) First Officer

You're confusing AGL altitude (above ground level) with absolute altitude (distance above sea level.)

Your altimiter displays your absolute altitude, which will almost always be greater than zero. (There may be an airport in the middle of death valley, but for the most part they're above sea level.) It's based on barometric pressure, which isn't affected by whatever patch of ground you happen to be sitting on. Try flying out of denver some time Smile

many planes also have a radar altimeter that gives you your altitude AGL.

Pro Member Chief Captain
Jonathan (99jolegg) Chief Captain

whittke1 wrote:

When I select a plane and the airport, my altitude is already at 400 sometimes. How can I reset this to 0?

Altimeter subscale settings (29.92 or 1013.25) reflect altitude above a given datum point.

When the altimeter shows you at 400ft...the current air pressure equates to 400ft above that of the datum pressure selected...assuming roughly 27ft per millibar.

Because QNH (altitude above mean sea level or AMSL) is commonly used, this would mean that you are 400ft above sea level as the pressure you are entering is the air pressure at sea level.

QFE (the altitude above a set datum point, such as an airfield) can be used whereby the air pressure of the airfield is used as the setting. Therefore, when at that airfield, the altimeter will stay at zero because there is little to no difference between barometric pressure of the datum point, and the current altitude.

Therefore, to change from QNH to QFE, you simply need to adjust the small knob on the altimeter to alter the altitude to zero...by doing this, you find the QFE subscale setting.

ATIS (automatic broadcast) will give you the pressure setting to use. This means all aircraft operating out of that aerodrome are using the same settings to avoid accidents.

Karlw mentions the transition altitude...whereby the altimeter subscale setting must be altered to one standard setting (29.92 or 1013.25) so that commercial traffic again, work from one setting irrespective of terrain passed in flight. However, it's important you note that the transition altitude varies from country to country and airspace to airspace. In the UK, TA is 3000ft but in some of the TMAs it is 6000ft so it really depends.

Pro Member First Officer
Jim Lapinsky (7ECA-Captain) First Officer

Suggest you review altimeter settings and the difference between agl, pressure, and how it affects the indications. If you are at an airport tht is at 400 feet AGL at standard altimeter setting, the altimeter will indicate 400 feet. while you can set the altimeter to 0 for that airport, it will need to be reset in flight, or even as pressures change on the ground without your aircraft even moving. This is why we have weather.

Pro Member First Officer
Jim Lapinsky (7ECA-Captain) First Officer

Sorry I should have explained better. I'm not a flight instructor, just an old real world pilot. Aviation altitudes are based on sea level. If you are cleared to 6000 feet it is above sea level (ASL) and not related to ground level (AGL). If you were to set your altimeter to the field elevation say 1000 feet the pressure scale will show the barometric pressure for that airport. the reverse is also true in that the correct barometric pressure will show the correct field altitude. This is a good way to check you altimeter for accuracy. If you are at a field that is at 400 feet, your altimeter should correctly read as 400 ft. When you climb to the indicated altitude of 2000 ft. your alimeter will read 2000 ft ASL (Above Sea Level) although your are in fact 1600 ft AGL. (Above Ground Level) Always fly the indicated level, or as it is known, pressure altitude. When you are approaching a controlled airport, on initial contact, they will give you winds and other information including the barometric pressure which you then verify on your altimeter and adjust as necessary. You always need to account for the field elevation so again if the airport is at 400 ft ASL and you are at 2000 and want to land, You would need to decend 1600 ft, not 2000. On landing the alimeter will read 400 ft the elevation of the airport. Because pressure changes, it would not work to set the altimeter to ground, as it can vary by hundreds of feet and the ground rises and falls so no setting woud be constant. For the purposes of flight, sea level is considered to be constant. Hope that helps and answers your question.

Cheers... Captain Scott

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