Converting QFE to QNH Rule of Thumb

[bluepilot76]

Now that we have accurate QFEs to use, it is useful to know that you can work out the QNH yourself from that, as long as you know the altitude of the airport.

 

Useful Rule of thumb: 1mb is 30 feet.

 

Divide airport altitude (in feet) by 30 feet. Add resultant number of millibars onto the QFE that you are given. Then you have QNH.

 

If you are doing Instrument approaches you would want to add a safety margin onto the DA because it is a rule of thumb, not 100% accurate.

 

For Example

airport 600ft amsl. QFE reported as 990mb.

 

600 divided by 30 equals 20. Therefore air pressure is 20mb lower than at sea level. QFE provided is 990mb, therefore QNH is 1010mb.

[effte]

Or you can subtract the aerodrome elevation from decision heights etc, which is bound to be more accurate. Just a thought...

 

Edit: On the plates I have from the Georgian AIP, the height above the threshold (as per above) is in the plates, so no need for math.

Edited June 6, 2011 by effte

[as179mike]

So where can you find out more about QFE? I'm always wondering what ATC is instructing me to do on TO. I have to admit I'm a privat pilot and I've never heard of QFE before.

[Pyroflash]

You can pretty much ignore the QFE on TO, as it is just an altimeter setting. On landings it might be a bit more useful to show the elevation of the airfield.

[as179mike]

Thanks for the info. After reading up on the q codes it would seem that I've been used to QNH. My guess is QFE is more relevant to IFR flight and DH on approach?

[Kiwispit]

QFE is used by the military. Gives zero altimeter at aerodrome.

 

QNH is used by civil aviation. Gives altitude referenced to Mean Sea Level (MSL)

 

IFR approach uses QNH. For example, runway might be 200' MSL, and DA/MDA might be 400' MSL then pilots would set QNH and fly to 400' MSL then continue if visual or go around.

 

Cat II and III approaches and autoland are referenced to Radar Altimeter but QNH is still set.

Edited June 7, 2011 by Kiwispit

[Kiwispit]

On 15 September 2003, a U.S. Air Force Thunderbird F-16C crashed during a Mountain Home, Idaho, air show. Capt. Christopher Stricklin attempted a "Split S" maneuver based on an incorrect mean-sea-level altitude of the airfield. Climbing to only 1,670 ft (510 m) above ground level instead of 2,500 ft (760 m), Stricklin had insufficient altitude to complete the maneuver, but was able to guide the aircraft away from the spectators and ejected less than one second before impact. The pilot survived with only minor injuries; the aircraft was destroyed. US Air Force procedure for demonstration "Split-S" maneuvers was changed to require pilots and air controllers to both work in above mean-sea-level altitudes.

[BlueRidgeDx]

QFE is not used by the military; at least not in the US.

[EtherealN]

Well Kiwi, that there is a very specific case where a ground reference is the one and only thing you want. Using QFE makes sense in that situation.

 

It would be interesting to find a list of which airforces use what and for what situations. Obviously though, everyone should be capable of using either. Hearing mentions of pilots who don't know what QFE is makes me scared. (I myself find most of my flying being done on QNH, but those two and how they work were constantly hammered into us on nav class.)

[Kiwispit]

I know our Air Force used to use it at flying displays but don't know if they still do.

 

I have never heard QFE being used in civil flying. I think one of the problems with QFE is the possible introduction of an error when working it out. Best if everybody uses the same datum.

[macedk]

well i look out the big bubble shaped window...oh hello mr.ground..

[Kaiza]

In the bottom part of the world QFE is used by the military for air displays only. In fact it is commonly incorporated into a "pre" and "post display" checklist, and some display directors will request a readback.

 

Youtube vids suggest that its used by US military for displays also.

[BlueRidgeDx]

Aerial demonstrations are a special case and the exception that proves the rule. They're worried about not hitting the ground within the airport boundary fence, so QFE is appropriate in that case.

 

In IMC, I'd be a bit more worried about running into high terrain or a vertical obstruction. In which case, those things are charted in MSL...not with reference to field elevation.

[bluepilot76]

The RAF uses it in the UK when giving the airfield information, if several bases are close together they provide a "clutch QFE" which is a single QFE used at all the bases. I suppose for a lot of the bases QNH are not that relevent as they probably depart straight onto a radar altimeter height.

 

At my local airfield, you tend to be given QFE if your flying something small and QNH if your flying commercial. Even though many pilots (myself included) would prefer the QNH, and try to instigate QNH when reporting in with the relevent airfield information, the controllers just start giving QFEs. Not a big deal though as my airfield is only 30feet off sea level..

 

We like to use as many different measurement systems as possible in the UK, keeps you on your toes!

[uri_ba]

If I know the airodrom alt I just dial in the correct MSL and QNH shows up. No need for math that might cause errors

[bluepilot76]

Yes the problem arises when you are returning from a mission, in IMC, and the air pressure has changed. The ATC provides you with QFE. If you WANT to use QNH then you need to know how to figure it out.

 

Dividing one number by 30 is not really complicated maths either? Pilots do little sums like that all the time, its all part of the simulation to me.

[VirusAM]

Now that we have accurate QFEs to use, it is useful to know that you can work out the QNH yourself from that, as long as you know the altitude of the airport.

 

Useful Rule of thumb: 1mb is 30 feet.

 

Divide airport altitude (in feet) by 30 feet. Add resultant number of millibars onto the QFE that you are given. Then you have QNH.

 

If you are doing Instrument approaches you would want to add a safety margin onto the DA because it is a rule of thumb, not 100% accurate.

 

For Example

airport 600ft amsl. QFE reported as 990mb.

 

600 divided by 30 equals 20. Therefore air pressure is 20mb lower than at sea level. QFE provided is 990mb, therefore QNH is 1010mb.

 

 

this is true but it is more accurate if you divide by 27, and in dcs a10 ATC gives the QFE in hp not mb, so how do you translate a QFE of 27.99hp to a QNH?

Edited June 8, 2011 by VirusAM

[bluepilot76]

Hi Vincent, I believe that mb are hPa are effectively interchangeable, sorry I should have written hPa but I am used to thinking mb. You are alse right that in the lower levels of the atmosphere, 1mb is actually 27 feet, but it is a rule of thumb, pilots use 30 because it is easier to calculate.

 

Rules of Thumb are widespread in aviation, they do not provide the exact answer but provide you with an answer that is accurate enough, quickly. Chances are you cant fly to the nearest foot anyway. If you really want to get precise you also have to take into account the temperature of the air, it all gets messy very quickly... meanwhile your plane is busting russian airspace. Its a trade off, how accurate do you need to be, versus keeping on top of all the other things that are happening as your aircraft is speeding around. If pilots were that fussy about exact numbers they probably wouldnt be pilots, maybe accountants lol.

 

EDITED TO ADD. Hi Vincent on re-reading your post it occurs to me that perhaps you are getting inches of mercury confused with hPa? Inches of mercury (used in the USA) are writted XX.XX for example 29.92in is standard pressure setting, which in mb( or hPa) is 1013. I dont know how you convert inches of mercury into height. However, I think you will find that in DCS A10C, the tower is providing air pressure information in mb (hPa).

Edited June 8, 2011 by bluepilot76

[AlphaOneSix]

this is true but it is more accurate if you divide by 27, and in dcs a10 ATC gives the QFE in hp not mb, so how do you translate a QFE of 27.99hp to a QNH?

 

From 0-5000 feet, you can use 1 inHg per 1000 feet. So if your airfield says the QFE is 27.99, and it's at 2,000 feet, then the QNH would be 27.99+2=29.99inHg. Above 5000 feet is starts getting wonky, but most airfields are below that (not counting the one I'm at right now which is like 5860 feet).

[bluepilot76]

AlphaOneSix,

Im really glad you explained that! inHg have always been a mystery to me!

[as179mike]

 

Really? I was always instructed in terms of AMSL and AGL not Q--. This was about 15 years ago so I'm not sure if the teaching terminology has changed since then.

[kimkiller]

QFE can be helpful for maintenance reasons as well.

Especially when performing powerchecks on engines.

[effte]

Really? I was always instructed in terms of AMSL and AGL not Q--. This was about 15 years ago so I'm not sure if the teaching terminology has changed since then.

 

QNH/QFE and altitude above MSL/AGL are two different things, even though using one in the first group will get you one in the other, so I would hope both are taught. :)

[redtail]

Really? I was always instructed in terms of AMSL and AGL not Q--. This was about 15 years ago so I'm not sure if the teaching terminology has changed since then.

 

same here. I went to flight school in New Jersey back in 1998 and got my PPL in 1999 and the only thing I was taught was MSL / AGL so when given an altimeter setting from the tower, I set that and read my altitude MSL. Never got any Q's

[VirusAM]

Hi Vincent, I believe that mb are hPa are effectively interchangeable, sorry I should have written hPa but I am used to thinking mb. You are alse right that in the lower levels of the atmosphere, 1mb is actually 27 feet, but it is a rule of thumb, pilots use 30 because it is easier to calculate.

 

Rules of Thumb are widespread in aviation, they do not provide the exact answer but provide you with an answer that is accurate enough, quickly. Chances are you cant fly to the nearest foot anyway. If you really want to get precise you also have to take into account the temperature of the air, it all gets messy very quickly... meanwhile your plane is busting russian airspace. Its a trade off, how accurate do you need to be, versus keeping on top of all the other things that are happening as your aircraft is speeding around. If pilots were that fussy about exact numbers they probably wouldnt be pilots, maybe accountants lol.

 

EDITED TO ADD. Hi Vincent on re-reading your post it occurs to me that perhaps you are getting inches of mercury confused with hPa? Inches of mercury (used in the USA) are writted XX.XX for example 29.92in is standard pressure setting, which in mb( or hPa) is 1013. I dont know how you convert inches of mercury into height. However, I think you will find that in DCS A10C, the tower is providing air pressure information in mb (hPa).

 

Yes you are right, i confused mesurement units.

Anyway with the new patch the tower is giving QFE informations in inches of mercury(that is why i used as example 27.99), so i found a website to that helps you do that convertions, but i would like also to use a similar rule of thumb

 

this is the website i use http://www.hochwarth.com/misc/AviationCalculator.html