Using the Altimeter

[Captain Orso]

I've only just realized that when you start on the ground, the altimeter is always set to 0 AGL (above ground level) by setting the reference pressure knob to the QFE/QNH the ATC reports.

 

This is kind of weird to me. Why should it not actually be set to show the actual altitude ASL?

 

It would actually give you more important information than AGL, which actually only pertains to that one airport. Just as a wild example, imagine you want to fly from Denver to San Francisco. Denver is already above 5000" ASL. So if you set your altimeter to the QFE at Denver International, when you are flying over the mountains, you either have to just eyeball it to tell if you are high enough to get over a pass or mountain, or you would have to calculate in your head: altimeter + altitude of Den-Int - mountain/pass height = clearance.

 

Also, once you start descending to land, your altimeter will actually drop below 0, because SF-Int is near SL.

 

Am I missing something here?

[NeilWillis]

Yes, you're missing the fact that all air traffic controllers know their local air pressure, and whenever you are inbound to an airport, the QFE (local air pressure) is given to the incoming aircraft.

 

Also, once airborne, at a fixed altitude, standard pressure is dialled in to all altimeters so that all aircraft, no matter where they took off from are flying with standard pressures, and therefore all flight levels are correct and matching. The alternative would be 2 planes from different locations flying on their local air pressures, and assuming a good vertical separation, could in fact be at the same height.

[Captain Orso]

If I understand correctly, once a specific altitude above the airport has been reached, the altimeter automatically switches itself to a nominal altitude display depending on the the barometric reading? I've never noticed that :huh:

 

Or does the pilot have to actively change the setting?

[Art-J]

I'd say pilots do it themselves, never heard of automation of this process, especially given the fact, that even though transition to 1013.25 HPa might be international standard, the transition altitude itself is different between countries, and even between local airports.

 

As for the DCS, the Caucasus map is so small, the elevation differences between airports are small too (they are bigger only when flying from northern part over the mountain to the southern part or the other way), there is no civil air traffic to avoid, most offline and online missions begin and end at the same airport, people tend to fly low anyway, ATC modelling is primitive to say the least... All and all, there's just no need to be bothered with different altitude types and transitions at the moment. Maybe there will be, when new maps and more detailed ATC comes some day.

Edited July 4, 2016 by Art-J

[saburo_cz]

Not automatically but pilots do (or did) it. I am not sure if it is still for modern planes, i remember if from older types, our pilots did it.

I do not know right English formulation for that altitude, only Czech.

 

btw. i would prefer altimeter set to actual altitude instead of "zero alt.", like, for instance, F-15C has it

[Flamin_Squirrel]

Modern airliners allow you to enter transition level/altitude prior to departure, but this only results in the aircraft prompting the pilots to make the change at the appropriate point, rather than make the change for them. I doubt any military aircraft (with the exception of civil derivatives) have this feature though.

[Captain Orso]

One of the very first long distance flights I made was from Tbilisi-Lochini to Kobuleti; that's going from 1550" to 60". It's not as great an altitude difference as Denver to San Francisco, but it's still 1500".

[BSS_Sniper]

Yes, you're missing the fact that all air traffic controllers know their local air pressure, and whenever you are inbound to an airport, the QFE (local air pressure) is given to the incoming aircraft.

 

Also, once airborne, at a fixed altitude, standard pressure is dialled in to all altimeters so that all aircraft, no matter where they took off from are flying with standard pressures, and therefore all flight levels are correct and matching. The alternative would be 2 planes from different locations flying on their local air pressures, and assuming a good vertical separation, could in fact be at the same height.

 

Well, not quite correct. Once you're at 18,000 then 29.92 is used, Below that you're using the local pressure.

[Art-J]

Well, not quite correct. Once you're at 18,000 then 29.92 is used, Below that you're using the local pressure.

 

Isn't it exactly what he said though, just without the numbers? On a side note, I believe 18k limit is used in the USA only.

[Holbeach]

QFE 0 ft. is set at the airfield for take off.

 

QNH is also given by ATC and is set by the pilot, after leaving the airfield and is used for terrain clearance. A new QFE will be given by ATC at the next landing airport.

 

Transition to flight level varies throughout the world and is dependant on local ground clearance. In North America it's standardized at 18000 ft, elsewhere it could be down to 3000 ft.

 

Flight levels on the standard pressure are completely irrelevant in this game.

 

..

[NeilWillis]

I didn't add any numbers to my post for the simple reason that it varies from airspace to airspace.

 

I was simply giving an outline of the principals and reasons. Getting bogged down in detail wasn't helpful, and the OP was asking quite a straightforward question, so he got a pretty simple, straightforward answer.

[Anatoli-Kagari9]

Beware that DCS also models very low temperature effects ( Winter scenarios ) of the geopotential height, so that for the exact same QNH, in cold Winter days you'll be ( MAGL ) lower than on a corresponding situation in Summer time!

[janpitor]

Gentlemen, just for clarification, I do not know about US, but in Europe QFE is very rarely used (maybe some local aeroclub flying). Normally QNH is set on the ground up to transition altitude. So OP's question is valid

[Flamin_Squirrel]

QFE is used routinely in the UK by GA and the RAF.

[Ala13_ManOWar]

Gentlemen, we are mixing things here and I'm not sure Captain Orso can easily deal with all the related to altimetry but unrelated to his question things said.

 

Let me give a try,

 

I've only just realized that when you start on the ground, the altimeter is always set to 0 AGL (above ground level) by setting the reference pressure knob to the QFE/QNH the ATC reports.

 

This is kind of weird to me. Why should it not actually be set to show the actual altitude ASL?

 

It would actually give you more important information than AGL, which actually only pertains to that one airport. Just as a wild example, imagine you want to fly from Denver to San Francisco. Denver is already above 5000" ASL. So if you set your altimeter to the QFE at Denver International, when you are flying over the mountains, you either have to just eyeball it to tell if you are high enough to get over a pass or mountain, or you would have to calculate in your head: altimeter + altitude of Den-Int - mountain/pass height = clearance.

 

Also, once you start descending to land, your altimeter will actually drop below 0, because SF-Int is near SL.

 

Am I missing something here?

To start with, setting altimeter to 0 which means the same of using QFE given by an ATC is a USA procedure and usual practice, but it isn't by far the only procedure nor the only World standard. For instance in the old Europe we use indeed always and without any doubt QNH (so altimeter shows airport elevation). That should tell something about your first question. And yes, anyway in cockpit you're resetting your altimeter (as long as gyrocompass and may be artificial horizon) every time needed, but it isn't any big deal holding the mouse to do so :smilewink:. One of the reason you can reset your altimeter may be reaching a new destination so ATC gives you current QNH (small differences usually but still...), or may be you...

 

Well, not quite correct. Once you're at 18,000 then 29.92 is used, Below that you're using the local pressure.

...reach Flight Level altitude so you start using universal atmospheric pressure, 1013.25mb, 29.92''Hg, 760mmHg, also called QNE, that means same reference for everybody up there. But following ICAO recommendations the lowest legal Flight Level can be 3500'' IIRC, but it depends on local aviation administration. For instance, here in Spain as we actually have the second higher medium ground altitude in Europe (after Switzerland) we have a minimum legal FL of 6500'' (or FL065) IIR, but, that means FL so if you are climbing from a QNH altitude that 6500'' (QNE) are indeed changing every day, so the legal levels real altitude referred to QNH (or when you reach transition altitude to FL) are daily published by local aviation administration.

 

In DCS as we have no civil ATC nor kinda FS yet there's no point IMO in using QNE at all, you can choose QNH (I think that's where altimeter appears as you click play) or setting your altimeter to 0 for a local flight if you don't plan to go to a different airport.

 

 

Hope that sheds a bit of light on your concerns.

 

S!

Edited July 5, 2016 by Ala13_ManOWar

[Art-J]

Gentlemen, just for clarification, I do not know about US, but in Europe QFE is very rarely used (maybe some local aeroclub flying). Normally QNH is set on the ground up to transition altitude. So OP's question is valid

 

If I recall correctly QFE is still standard in Russia (as it was in USSR and all Warsaw Pact countries till the end of Cold War), they've started adopting QNH in civilian aviation only recently, so one might argue that QFE is quite "correct" on our Caucasus map. Not on Nevada, though!

[Ala13_ManOWar]

If I recall correctly QFE is still standard in Russia (as it was in USSR and all Warsaw Pact countries till the end of Cold War), they've started adopting QNH in civilian aviation only recently, so one might argue that QFE is quite "correct" on our Caucasus map. Not on Nevada, though!

It is not correct or incorrect, it's a matter of being used to one or another. I'm not used to QFE, I'm used to QNH and I prefer that so I can read real altitudes in a map, cross myself with another traffic without concerns about its real altitude or go a different airport just knowing it's height (published in every chart). Others may be used and prefer QFE because altimeter says 0 when you land (like you were looking to your altimeter while landing :lol:). But there's no correct one :smilewink:.

 

S!

[Flamin_Squirrel]

To start with, setting altimeter to 0 which means the same of using QFE given by an ATC is a USA procedure...

 

Don't believe QFE is used at all in the USA. When I flew out there, my instructor had never even heard of it.

 

...reach Flight Level altitude so you start using universal atmospheric pressure, 1013.25mb, 29.92''Hg, 760mmHg

 

Corrected.

[Ala13_ManOWar]

Corrected.

Thanks!! my mistake :smilewink:.

 

 

Don't believe QFE is used at all in the USA. When I flew out there, my instructor had never even heard of it.

Don't go so far, if your instructor don't know what QFE is may be he shouldn't be an instructor at all... May be they use a different name (I don't think so but who knows), or whatever reason, but you use it or not it's pretty basic theory. Is he an old man perhaps?

 

In aviation terminology, the regional or local air pressure at mean sea level (MSL) is called the QNH or "altimeter setting", and the pressure that will calibrate the altimeter to show the height above ground at a given airfield is called the QFE of the field.

 

 

S!

[Flamin_Squirrel]

No, lady in her 30s. Don't be so surprised, it might be standard theory, but aviation isn't as ridged as one might think.

 

 

...also, this was in Arizona with high altitude airfields. Using QFE there isn't practical as it would take so long turning the dial from QNH!

[Ala13_ManOWar]

No, lady in her 30s. Don't be so surprised, it might be standard theory, but aviation isn't as ridged as one might think.

 

 

...also, this was in Arizona with high altitude airfields. Using QFE there isn't practical as it would take so long turning the dial from QNH!

Well, may be it isn't in USA, but still it's pretty basic theory I had to demonstrate knowledge of in my own PPL exams :lol:, not just questions, you're even required to solve a good bunch of altimetry problems regarding the subject in the exam. May be she didn't recall for a reason, but I seriously doubt she don't know what QFE is :smilewink:.

 

The use of QNH in a high altitude area makes sense, but still QFE use is quite an american think.

 

S!

[Captain Orso]

Wow, thanks for all the replies guies.. I mean guys :smilewink:

 

So, I was googling for a simple formula to calculate the QNH from the QFE, and first you need the true altitude of the runway, and then after searching around further I realized the simplest method would be to adjust the inHg in the Kollsman window in the altimeter until the altimeter reads the true altitude :doh:

 

It's not a pretty method, but simple :D

[Ala13_ManOWar]

Wow, thanks for all the replies guies.. I mean guys :smilewink:

 

So, I was googling for a simple formula to calculate the QNH from the QFE, and first you need the true altitude of the runway, and then after searching around further I realized the simplest method would be to adjust the inHg in the Kollsman window in the altimeter until the altimeter reads the true altitude :doh:

 

It's not a pretty method, but simple :D

And that's exactly the kind of altimetry problems you're requested to solve in your PPL :smilewink:, of course without using the altimeter :D. QNH should be stated somewhere, that should be the map pressure and may be you find it in the briefing even though in mm Hg, or milibars. The other way is knowing your QFE and airport altitude as you know pressure descends, 35mb/1000ft? I don't recall right now, but you get the idea, so you can calculate QNH quite easily or whatever you want to know.

 

Another one would be, but a bit less reliable I believe, using manifold pressure indication before starting the engine. That's the QFE indeed :thumbup:.

 

S!

[BSS_Sniper]

Isn't it exactly what he said though, just without the numbers? On a side note, I believe 18k limit is used in the USA only.

 

You said once aircraft are at a fixed altitude, I was just adding that fixed altitude is 18k and above.

[Captain Orso]

And that's exactly the kind of altimetry problems you're requested to solve in your PPL :smilewink:, of course without using the altimeter :D. QNH should be stated somewhere, that should be the map pressure and may be you find it in the briefing even though in mm Hg, or milibars. The other way is knowing your QFE and airport altitude as you know pressure descends, 35mb/1000ft? I don't recall right now, but you get the idea, so you can calculate QNH quite easily or whatever you want to know.

 

Another one would be, but a bit less reliable I believe, using manifold pressure indication before starting the engine. That's the QFE indeed :thumbup:.

 

S!

 

Well, yes, I did find lot's of formulas to convert, but most of them required that fancy-shmancy heck-toe-pastel thingy hPA , and I wasn't interested in introducing converting inHG to hPA to that too :huh:.

 

Some of the equations I was looking at are probably good enough for the renowned PPT, and require season, temperature, wind-strength, and the color of your girlfriends thong on flight-day . Just a bit too much overkill for the job.

 

I suppose I could always use the MP at stopped engine, although it's not really as exact as the Kollsman setting, but since I'm not expecting to do any instrument flying, I don't suppose I'll get so confused by a few feet difference between visual and altimeter that I'd be in danger of hitting the ground... I think... maybe... :joystick:

Edited July 6, 2016 by Captain Orso