QFE on a slope runway?

[Aceostrat]

I am just wondering that on such a slope runway while taking off, which position does the QFE read '0' when you are on it?(the highest point on the runway(AAL) or its departure end elevation?)
Since I have been thru so many people done the settings regarding the acceleration altitudes which someones use an altitude + a departure end elevation and others use + AAL.
Could someone please clarify this? and I would really appreciate a true reference link for it.

[FlightDetent]

The definition you seek may not exist.

As a somewhat helpful reference, remeber that for NPAs the OCH is calculated against the field elevation, unless the threshold is lower up by 7 feet or more.

The software that calculates RTOW charts will have a method to slove this, although it may differ among the providers. DERA makes little sense, btw.

On practical terms, in daily life I use THR ELEV as reference zero. The way it comes from the FMS after selecting DEP runway.

[Check Airman]

Interesting question, but I'd suggest that if you're taking the runway slope into account for your acceleration altitude, you either shouldn't be flying, or shouldn't drink as much coffee.

[Miles Magister]

Whilst I do not have the references to hand, I believe that the QFE is measured for the runway threshold. You could have different QFE values for different runways at the same aerodrome. Back in the day when I was fortunate enough to be taught by the Queen's finest we always did the altimeter check at the threshold.

Edited to add;
ICAO Doc 8896
Note.— When a QFE altimeter setting is provided, it corresponds to the aerodrome elevation except for:
a) non-precision approach runways, if the threshold is 2 m (6 ft) or more below the aerodrome elevation; and
b) precision approach runways;
in which cases, the QFE corresponds to the relevant runway threshold elevation.

I hope this helps you.

[Checks Complete]

It's all covered in BCARs

[22/04]

In the days when QFE was widely used some airfields with runways where the threshold was significantly different from the airfield sued to provide a threshold QFE - Luton was one of them.

In these days of widespread adoption of QNH this has become unnecessary.

[FullWings]

Agreed. It’s not a stupid question (genuine ones rarely are) but as the old saying goes: ‘perfection is the enemy of good enough’. There are plenty of rabbit holes like this in aviation where it is possible to completely overthink a problem (or even to think there is a problem in the first place).

What are you trying to prevent and how is it defined? What difference would it make if you did it either way? If it isn’t an SOP there is likely a reason for that.

Fixating on trivia can have a negative effect in the real world. At the end of the day, there are tolerances and margins in everything and the difference in elevation between various bits of a runway is lost in the noise when it comes to practical aviation (unless you’re in Alpe d’Huez or Ulanbataar). Having spent tens of thousands of hours in the air, my understanding of the idea is to make it as simple as possible, and to avoid useless layers of complication and personal quirks that confuse anyone you fly with. Spending your limited cognitive resources on things that have no measurable impact on the operation means you have less to cover the important stuff, like which way to turn if you lose an engine...

[DaveReidUK]

ICAO PANS-ATM:

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/998x510/icao_doc_4444_4037cd757c61c2a7ad7346b9ce6926b30bda0156.jpg

[jmmoric]

Now I get curious...

According to annex 4, an aeronautical chart shall show "geoid undulation" for thresholds...

I know the difference between the WGS84 geoid and local mean sea level is known as "geoid undulation", and that is relevant when flying using geometric VNAV, which is a requirement when flying LPV approaches. The threshold height in reference to the geoid can be seen in the FAS data block for LPV procedures.

The threshold elevation in reference to the geoid is then the geoid undulation for a threshold + the elevation for the threshold in relation to mean sea level.

But honestly, do anyone ever use this data when flying? Besides when programming the FMS (which I suspect is done by technicians before flight).

[Level bust]

Luton used to publish 3 QFEs. One for the airfield and then one for each threshold. I can't remember now if we added or subtracted the threshold QFE from the airfield one. I know it was 0.4 and 0.7, so occasionally one threshold QFE was different to the other as a result of always rounding down.

[DaveReidUK]

For reference, Luton's thresholds are 11 feet (07) and 18 feet (25) lower than the airfield reference point (runway midpoint).

[ShyTorque]

For aircraft performance calculations, surely density altitude is what counts. At least it does on the types I've flown.

Altimeter settings for an instrument approach are all about knowing how close the runway surface is, a different exercise.

[jmmoric]

Agreed, but not when you fly an LPV approach, then the VNAV is not barometric, but geometric.... and geometric can also be used for LNAV/VNAV when temperature drops below the minimum for BARO-VNAV.

But you know way more about this topic than I do.... I'm just trying to suck up as much information as possible, and incorporate it into ATC training.

[Aceostrat]

FullWings

The reason for this question is nothing but to make and get it right every time I fly, just for the reason behind things. I've seen so many people's discrepancy over this issue, some set the acceleration altitude eg. 1000' + AAL, the others set 1000'+departure end elevation, and so forth (sometimes, in some airports, it's quite different).

We've been flying for so long with somethings as vague and undefined with so many improper explanations, on such questions as this one that WHY do we have to add it the way it is.

and I think everyone here could definitely help ;-)

[Aceostrat]

Thanks you all and DaveReidUK for the ref, . So is this applied as well on departure? and what would you set in the acceleration altitude field when you fly? eg. altitude + AAL or + departure runway threshold or + opposite runway threshold.

Miles Magister

Thank you, this really helped. So is this applied as well on departure? and what would you set in the acceleration altitude field when you fly? eg. altitude + AAL or + departure runway threshold or + opposite runway threshold.

[FlightDetent]

Depends on the question,
​​​​​​
​​​​​​. normal proc, noise abatement procedure, OR
​​​​​​. engine out safety profile.

​​​​​​?

[FullWings]

Aceostrat

What does it say in your Performance Manual? That is the ultimate practical authority.

Not all questions in aviation actually have a precise and/or useful answer and often require the use of common sense to fill in the gaps. What would you set as an AA with an intersection departure? Do you need to correct for humidity and local gravity anomalies? Does a jet aircraft have a critical engine?

If everyone you fly with does a particular thing slightly differently, including trainers, then it probably doesn’t matter. If there is a universally adhered-to standard, then it probably does.

A really good explanatory document is Getting to grips with aircraft performance (https://www.skybrary.aero/bookshelf/books/2263.pdf). It comes from Airbus but 99% of it is applicable to any aircraft, and gives an excellent insight into why we do what we do most of the time...