charts Questions regarding RWY altitude

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Hi,

Cany anyone enlight me what the /hectopascal information is given/used for in the below image at  ? (in this case EPWA)

Thanks

Michael Moe

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The pressure changes (decreases) with altitude -- this is how an altimeter works, by measuring the actual pressure and comparing it to the datum set in the window. This works because the pressure reduces at a variable with altitude, but known rate -- at sea level, 1hPa = approximately 30ft (actually about 27, but it changes with height and 30 is close enough for flight deck work and a nice round number to do mental maths with).

So you can express an altitude either in terms of feet, or in terms of the ISA air pressure at that level (so, for example, the air pressure at 10,000ft is 700hPa according to the ISA and you will sometimes see weather charts that express altitude in terms of pressure (hPa) rather than units of length (feet)).

Thus, a change in height of 346ft is equal to a change in pressure of 13hPa (346/27 = 12.8). This is particularly useful to a pilot who wants to set QFE on the altimeter for landing: the QFE is the QNH adjusted for the elevation of the threshold. So if the QNH is 1013hPa, setting 1000hPa (1013 - 13) will make the altimeter display height above the threshold during the approach and landing. Displaying the threshold elevation in terms of hPa on the chart does 50% of the sums for the pilot, so all they have to do is take that value and subtract it from the QNH.

QFE is hardly ever used in modern commercial transport operations (FMS systems are normally designed with QNH in mind and setting QFE confuses them), but GA and the military (read: RAF) use it extensively as many of their procedures are based on height, not altitude.

• 1

Hi,

Cany anyone enlight me what the /hectopascal information is given/used for in the below image at  ? (in this case EPWA)

Thanks

Michael Moe

As has been explained above the 13hpa is what you would "deduct" from the QNH to obtain QFE. Also to remind you the differences in measurements as they are significant. You mentioned runway "altitude". The meaning of the word altitude is "above sea level". So that is always expressed as QNH. That means that the airfield "altitude" would change every day according to the pressure at sea level. Obviously that doesn't happen! Because instead one uses airfield "elevation". The physical height above sea level the same measurement one uses for mountains etc. In aviation the word "height" is used to express "above ground". So if I am at a "height" of 1,000ft that means I am 1,000ft above ground level. The ground level itself of course can be at any elevation.

If you fly older aircraft without FMCs you will need to use QFE. The sim calculates one millibar i.e. 1 Hpa as 29.54545 feet. So in your example 13Hpa would be the equivalent of 384.09 feet (in the sim). But always allow for a discrepancy of 1mb and at 8,000ft allow a discrepancy of 3mb.

And when setting millibars using a kohslman knob the convention is to adjust it to the setting below the required and adjust up until the veeder counter changes to the required setting. So your altimeter QNH setting is at the bottom end so to speak.

• 1

If you fly older aircraft without FMCs you will need to use QFE.

Old/new, FMC equipped ir not has nothing to do with whether qfe is used. That is a procedural or regulatory preference based on perceived needs and convenience of an operator. eg. operators that routinely engage in aerobatics at low level may prefer to use qfe around an airfield or operators who primarily employ former military pilots may prefer to use qfe for familiarity, etc. However there is nothing technical that determines whether an aircraft is required to use it.

I was discussing the Flydubai accident with one of our former Emirates 777 jockeys last week and he was telling me that Russian airfields often use QFE. Combine that with using metres in an aircraft that is designed to be flown using feet, a very long sector, darkness, icing conditions, holding, fatigue and you can see that the holes in the Swiss cheese begin to align.

Russia does use metres QFE below TA, but will provide QNH on request and whilst I can't speak for Flydubai, at a Big Airline QFE operations are verboten so it is normal to request and set the QNH (heights will still be given in metres QFE by the controller but the approach chart includes a metres QFE --> feet QNH conversion table).

That said, it would be much easier if the Russians would come in to line with the rest of the world, and I suspect as time moves on they will move in that direction: my understanding is that the current situation (also in China with metric) is due to the fact that many older Russian-manufactured aircraft, and particularly military aircraft, are simply not equipped with altimeters calibrated in feet and so the system exists to cater for them. With more Western aircraft being operated by Russian operators, and Russian manufacturers designing aircraft for a global market, I imagine the situation will change sooner or later.

Russia does use metres QFE below TA, but will provide QNH on request and whilst I can't speak for Flydubai, at a Big Airline QFE operations are verboten so it is normal to request and set the QNH (heights will still be given in metres QFE by the controller but the approach chart includes a metres QFE --> feet QNH conversion table).

That said, it would be much easier if the Russians would come in to line with the rest of the world, and I suspect as time moves on they will move in that direction: my understanding is that the current situation (also in China with metric) is due to the fact that many older Russian-manufactured aircraft, and particularly military aircraft, are simply not equipped with altimeters calibrated in feet and so the system exists to cater for them. With more Western aircraft being operated by Russian operators, and Russian manufacturers designing aircraft for a global market, I imagine the situation will change sooner or later.

That's interesting stuff, thanks. I imagine in a high workload situation the whole converting metres/feet/QFE/QNH thing isn't much fun!

That's interesting stuff, thanks. I imagine in a high workload situation the whole converting metres/feet/QFE/QNH thing isn't much fun!

On the Super VC10 upgrade the PO automatically does the conversion for the QFE of the landing airfield.

Thanks alot,

just to sum it up a bit.

is it correct understood that for instance this metar in EDDF for runway 25R (TDZ353/13HPA)

EDDF 060750Z VRB02KT 9999 FEW025 18/13 Q1027 NOSIG

the QFE is 1014 ? So an altimeter/barometer calibrated at EDDF would show 1014=0 feet above AGL at the beginning of runway 25R ?

Thanks Michael Moe

Thanks alot,

just to sum it up a bit.

is it correct understood that for instance this metar in EDDF for runway 25R (TDZ353/13HPA)

EDDF 060750Z VRB02KT 9999 FEW025 18/13 Q1027 NOSIG

the QFE is 1014 ? So an altimeter/barometer calibrated at EDDF would show 1014=0 feet above AGL at the beginning of runway 25R ?

Thanks Michael Moe

It depends on the actual elevation shown on your afcad. The latest chart available on Eurocontrol shows 353ft elevation for that runway end. The sim calculates millibars at 29.54545ft  per millibar so 353 div 29.54545=11.94769

So 1027-11.94769=1015Mb. You are allowed a discrepancy of 1 Mb up to 8,000ft.

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