Difference between MSL calculations and Altimeter setting
Posted by iitscrouton@reddit | flying | View on Reddit | 25 comments
Hello, I am a bit confused about what we use to set our altimeters. I understand that when MSL is calculated it takes the average temperature at the location from the past 12 hours. However is it that we don’t bother with this calculation in our altimeter settings? I have been told that the altimeter setting is in relation to standard sea level setting of 15 C with a lapse rate of 1.98. However, wouldn’t using the standard atmosphere cause errors? Wouldn’t it be better to use the actual calculated MSL? Not sure what is more accurate. In the FTGU at 6.3.2 it says “in reducing station pressure to sea level pressure for altimeter setting purposes, the standard sea level pressure for altimeter setting purposes is used in computing equivalents.” I’m not sure exactly what it means by equivalents however.
iitscrouton@reddit (OP)
Alright guys I’m basically asking what the point of calculating MSL is if we don’t use it. Sorry for the wordiness ig
jet-setting@reddit
That still doesn’t clear up anything. We don’t calculate MSL, our altimeters indicate MSL when properly calibrated using the local altimeter setting.
I’m confused why you think we don’t use MSL? All terrain/obstruction elevations and airspace altitudes depicted on your charts are MSL. Instrument approaches are MSL, basically everything we do is MSL unless you’re in the flight levels or flying at an airshow.
Are you perhaps confusing MSL altitude with density altitude?
iitscrouton@reddit (OP)
It’s based on what the from the ground up is saying “the reduction to MSL pressure involves adding to the station pressure the weight of an imaginary column of air extending from the station level down to mean sea level. In determining MSL pressure, local temperature must be taken into account. The temperature value is based on the average of the surface temperatures at the time of observation and 12 hours before the time of observation.” From my understanding this MSL would differ from the standard atmosphere. So I’m asking why we use standard atmosphere instead.
Sensitive_Point_6583@reddit
I think you may be mis-using terminology a bit, and that's what's causing confusion in the replies.
when you say "calculate MSL" what you're really referring to, as you state in the paragraph above, is "barometric pressure corrected to sea level", or "MSL pressure". That's something that's of interest to weather people, but irrelevant to flying an aircraft.
As everyone knows, the altimeter setting used to calibrate an aircraft altimeter to runway elevation is based on the ISA lapse rate, and everyone's altimeter is calibrated the same way. So, it doesn't really matter if that altimeter setting matches the true "barometric pressure corrected to sea level" or not, what matters is that every aircraft is calibrated to the same standard. That guarantees that even though they may all be wrong, they're all wrong by the same amount, so everyone agrees on what the altitude is, even if its wrong. And, as has been said before, margins are built into the system so that its not critical to know the precise elevation, because its a known fact that ISA calibrated altimeters will always have some inherent error.
I'm not sure why the MSL pressure is even provided by airports, its not needed for flying. I'm not sure exactly which calculation you're referring to, but I think its the one called "SLP" in METARS reports, right? That calculation uses a hypothetical column of air from sea level to the airport, and uses the average temperature of that air column to calculate a pressure delta from sea level to the runway elevation. It then adds that pressure delta to the absolute air pressure at the airport to compute the SLP value. Here's an article that basically says what I've already said, but maybe it would help to read it.
https://www.wingsbywerntz.com/520-metar-slp-sea-level-pressure
Now, one other thing about the SLP calculation, its not normal for a column of air to have a constant temperature, but the SLP calculation assumes that it does. If the station elevation is relatively close to sea level, then the error introduced by that simplification isn't too great. At higher elevations, then it could become a large source of error. I don't know if they have a special formula to account for that or not, but I'd be interested in hearing about it if someone knows the answer to that question.
iitscrouton@reddit (OP)
Thank you for your response! Yeah I think I was using the wrong terms you are correct.
pronghornpilot@reddit
So, I got completely hung up on the idea you are quoting about “the reduction to MSL… …12 hours before the time of observation.” I had absolutely no idea what that’s from. I did some searching and it looks like that has to do with a method for comparing measured values of pressure between various weather stations. That has nothing whatsoever to do with altimeters and how to calculate your height above MSL. That method isn’t calculating height above MSL at all; it is baselining different weather station measurements and is predicated on already knowing the height above mean sea level of the stations.
iitscrouton@reddit (OP)
Could you send me some of the links that you looked at because I think this is the thing getting me confused 😭😭
pronghornpilot@reddit
I just googled “12 hours before the time of observation.” Make sure to put it in quotes. Only a handful of results surfaced and none look particularly good. Where did you get the quote from? It doesn’t seem to relate to flying in any way shape or form. If you are concerned about mean sea level in the context of being a pilot then abandoned the idea of calculating the location of mean sea level. Pilots do not do that. It is completely irrelevant to knowing your altitude relative to the generally accepted mean sea level. Independent-Reveal86 had a great answer.
iitscrouton@reddit (OP)
It’s from the From the Ground Up textbook
AlexJamesFitz@reddit
I may be totally misunderstanding, but it seems like you're mixing stuff up in a way that's making it difficult for folks to answer your question. Standard atmosphere is a unit of pressure, MSL is a measure of altitude.
pronghornpilot@reddit
Knowing the MSL of your aircraft lets you look at a chart and know if you are clear of terrain and obstacles among other uses. I don’t know why you think MSL isn’t useful. Also, from your post, I can’t tell how you are coming up with MSL at all.
iitscrouton@reddit (OP)
I guess I’m asking based on the image from this link here. Like the difference between the 1013.2 datum and MSLhttps://skybrary.aero/articles/altimeter-pressure-settings
pronghornpilot@reddit
Ok, that’s a good article. It is just talking about QNH vs QFE and general errors inherent to altimeters. How and why are you calculating MSL?
Put the QNH for a near by airport into your altimeter. When you are on the ground at that airport, your altimeter reads MSL. If you are flying over the airport, your altimeter will tell you an altitude very close to the MSL you are flying at. It’ll be a little bit wrong, but unless it’s wildly warm or wildly cold, it will be close to the right number. You don’t need to calculate a more accurate MSL from that value. Just know that if it’s colder than expected, you will be a little lower than you think. And on the flip side, if it’s warm, you’ll be a little higher.
Mispelled-This@reddit
You are seriously overthinking this to the point I don’t even understand your question.
Pilots don’t “calculate” MSL. We hear the altimeter setting, turn the dial on the altimeter to match, and then read MSL off the hands/tape. That’s it.
Yes, there is some error in the altimeter due to temperature, humidity, etc. That’s one reason why minimum altitudes have a huge safety margin. You’ll learn more about the limits of that during instrument training, but for now, just look out the window and don’t hit stuff.
iitscrouton@reddit (OP)
It’s not that pilots calculate it but I believe meteorologists do for weather trends
natbornk@reddit
Weather trends are great but that’s hard to make the connection to needing to know this to correctly anticipate altimeter errors in VFR conditions.
To elaborate on temperature errors, as long as your temperature is above like -30 (are you really flying colder than that?) and below like +50 (again, would you really be in a plane flying if it was hotter than that?) don’t worry about it at all
makgross@reddit
Because your altimeter is a barometer. It doesn’t measure altitude, but rather air pressure. The conversion to altitude it displays is exactly the ISA. It also has a single calibration, rather than a different one for every possible station altitude.
Independent-Reveal86@reddit
You seem to be overthinking this to the extreme.
All the stuff about calculating MSL in a standard atmosphere is built into the altimeter itself. It has to be calibrated to something and that something may as well be the standard atmosphere, otherwise why even have a standard atmosphere? It would be needlessly complicated to have an altimeter that could be programmed for something other than the ISA and it would need to be changed frequently because the atmosphere is always changing from moment to moment. The lapse rate now is not the same as the lapse rate in an hour’s time. The temperature outside of cloud is normally warmer than inside because of the different lapse rates in dry and saturated air.
It only has to be good enough to help us avoid hitting the ground and other aircraft. It doesn’t need to be perfectly accurate to the conditions now. It’s ok for the altimeter to be wrong as long as it’s the same wrong for everyone in the area and it’s not so wrong that it puts you into the ground.
iitscrouton@reddit (OP)
I think this makes the most sense thank you
phliar@reddit
Think of the altimeter setting as whatever you need to dial into the instrument so that it reads airport elevation on the ground. You only need to apply temperature etc. corrections if you are not on the ground at the airport.
DicksMcgee02@reddit
I have zero clue what I just read…
AlexJamesFitz@reddit
I'm confused by the question, but just use whatever setting the ATIS/minute weather/ATC gives you.
Yesthisisme50@reddit
I’ve read OPs post 3x and I have no idea what they even said lol
AlexJamesFitz@reddit
https://youtu.be/Ccoj5lhLmSQ?si=IZTKQmqN3g3lGyWR
rFlyingTower@reddit
This is a copy of the original post body for posterity:
Hello, I am a bit confused about what we use to set our altimeters. I understand that when MSL is calculated it takes the average temperature at the location from the past 12 hours. However is it that we don’t bother with this calculation in our altimeter settings? I have been told that the altimeter setting is in relation to standard sea level setting of 15 C with a lapse rate of 1.98. However, wouldn’t using the standard atmosphere cause errors? Wouldn’t it be better to use the actual calculated MSL? Not sure what is more accurate. In the FTGU at 6.3.2 it says “in reducing station pressure to sea level pressure for altimeter setting purposes, the standard sea level pressure for altimeter setting purposes is used in computing equivalents.” I’m not sure exactly what it means by equivalents however.
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