Quick Jepp question
- Thread starter CAFFEINE
- Start date
Chris_Ford
Well-Known Member
Don't worry, you don't need to know. A friend asked that exact question on the XJT interview and the interviewer did not know. However, the answer has to do with air pressure (NOT altimeter) at the airport elevation. Thus, you can reverse engineer to find elevation. However that's pure conjecture based on science type learning that I recall.
eagledriver101
New Member
Even though I don’t have clue about what you’re talking about, I have to agree. :yeahthat:
SIUav8er
Narcosis
hmm, i would guess that they use MB instead of inches of HG to measure barometric pressure (altimeter setting). Not sure what the TDZ MB thing is though, thats kinda weird. Its like saying the TDZE is 29.92 in. HG.
Chris_Ford
Well-Known Member
And what's wrong with saying that?
Rocketman99
Frozen Guppy Manipulator
Maybe the second half says this is an airport that intends for you to have your altimeter set so that on the ground it will read 0 feet (setting QFE instead of QNH) or something along those lines. This is not particularly uncommon in other countries. Along that then to figure out the actual field elevation in feet you would do a little mathemagics. My guess.
I would love to know the actual answer.
I would love to know the actual answer.
Chris_Ford
Well-Known Member
Okay let's try this again.
ALTIMETER SETTING DOES NOT EQUAL PRESSURE OUTSIDE OF THE AIRPLANE.
Altimeter setting is adjusted based on field elevation.
29,92" Hg = A29.92 only at sea level (given standard temperatures. Non standard temps at other altitudes would also permit this).
For example, as a rule of thumb, you lose an inch of mercury every 1000 feet.
In Denver, when they get the barometric pressure, it might actually be something along the lines of 24.92. Via a magic formula, they recalculate it and you'll get a local altimeter setting.
So where does this all come in to play? Let's take that 143MB and subtract it from standard pressure (1013.2 MB). We are doing this because this airport is 143MB higher than sea level. Through the magic of science, we're able to determine that 870.2MB is roughly equivalent to 4150 feet.
Does that correspond with the airport elevation (fairly closely)? If not, I messed up. If so, I did well
ALTIMETER SETTING DOES NOT EQUAL PRESSURE OUTSIDE OF THE AIRPLANE.
Altimeter setting is adjusted based on field elevation.
29,92" Hg = A29.92 only at sea level (given standard temperatures. Non standard temps at other altitudes would also permit this).
For example, as a rule of thumb, you lose an inch of mercury every 1000 feet.
In Denver, when they get the barometric pressure, it might actually be something along the lines of 24.92. Via a magic formula, they recalculate it and you'll get a local altimeter setting.
So where does this all come in to play? Let's take that 143MB and subtract it from standard pressure (1013.2 MB). We are doing this because this airport is 143MB higher than sea level. Through the magic of science, we're able to determine that 870.2MB is roughly equivalent to 4150 feet.
Does that correspond with the airport elevation (fairly closely)? If not, I messed up. If so, I did well
Rocketman99
Frozen Guppy Manipulator
We may be talking about two different things here Chris but along the right path. I'm saying this airport may be providing information for people using QFE altimeter settings.
So I dug out my jepps and looked up the approach chart legend info section.
The first half, "Alt Set: MB (IN on req)," means that the altimeter setting in use for the airport is given in millibars and inches are avilable on request.
The second half "TDZ Elev: 159 MB " from the explanation is "Barometric Pressure Equivalent in MB or hPa enables acft operators who use QFE altimeter settingfor landing to establish the QFE altimeter setting by subtracting the hPa or MB equivalent from the reported QNH altimeter setting. The value shown is the barometric pressure equivalent for the height reference datum for straight-in landing. The height reference datum will be the runway threshold elevation (Rwy), airport elevation (Apt), or the runway touchdown zone elevatio (TDZ), as applicable.
IOW, the note is there for people using QFE settings (where the airport altitude shows up as 0 on your altimeter) to convert the setting given by the controller in QNH (where the airport altitude shows up on your altimeter as the MSL altitude of the airport). Which is what I was talking about in my first reply, only I thought it meant the airport was giving QFE readings for you to convert from QNH and it's actually the other way around.
Hope it helps.
So I dug out my jepps and looked up the approach chart legend info section.
The first half, "Alt Set: MB (IN on req)," means that the altimeter setting in use for the airport is given in millibars and inches are avilable on request.
The second half "TDZ Elev: 159 MB " from the explanation is "Barometric Pressure Equivalent in MB or hPa enables acft operators who use QFE altimeter settingfor landing to establish the QFE altimeter setting by subtracting the hPa or MB equivalent from the reported QNH altimeter setting. The value shown is the barometric pressure equivalent for the height reference datum for straight-in landing. The height reference datum will be the runway threshold elevation (Rwy), airport elevation (Apt), or the runway touchdown zone elevatio (TDZ), as applicable.
IOW, the note is there for people using QFE settings (where the airport altitude shows up as 0 on your altimeter) to convert the setting given by the controller in QNH (where the airport altitude shows up on your altimeter as the MSL altitude of the airport). Which is what I was talking about in my first reply, only I thought it meant the airport was giving QFE readings for you to convert from QNH and it's actually the other way around.
Hope it helps.
FlyChicaga
Vintage Restoration
The answer is that every airport we fly to gives us altimeter settings in inches of mercury.
Airports in Europe give altimeter settings in mb, in fact, we go to 1013 at the transition altitude (usually 4 to 6K). Weird saying "Flight Level 090" but you do.