runway level

[tarbat]

I'm probably out of my depth here, but don't pilots use QFE at low altitudes to set the altimeter, so that their altimeter reads zero when on the deck.  Or do pilots always use 1013.2 to set their altimeter, even at low altitudes?  And is this the same altitude transmitted on ModeS/ADS/B?

Surely most aircraft can't determine an aititude without the pilot entering an air pressure?

[DaveReid]

I'm probably out of my depth here, but don't pilots use QFE at low altitudes to set the altimeter, so that their altimeter reads zero when on the deck.

Yes, that's precisely the purpose of QFE.

Or do pilots always use 1013.2 to set their altimeter, even at low altitudes?

1013.2 is the setting that Flight Levels are based on, so it's normally only set above the transition altitude (unless of course the QNH on the day happens to be this value).

  And is this the same altitude transmitted on ModeS/ADS/B?

The altitude transmitted on Mode S/ADS-B is always based on 1013.2 and not on the altimeter setting, so above the transition altitude it will be equal to the Flight Level.  Below the TA it won't be a true altitude AMSL unless the QNH also happens to be 1013.2

Which is where we came in ...

Surely most aircraft can't determine an aititude without the pilot entering an air pressure?

Quite so - they can determine what Flight Level they are at, but not what altitude.

[tarbat]

The altitude transmitted on Mode S/ADS-B is always based on 1013.2 and not on the altimeter setting

That's the vital bit of information I've been looking for - thanks.  But, does this also apply to military fast jets (eg the Tonkas)?

[DaveReid]

That's the vital bit of information I've been looking for - thanks.  But, does this also apply to military fast jets (eg the Tonkas)?

I don't know, but I can't see any reason why it would be any different.

Apart from us, I don't thing there is any use being made of Mode S altitude data from low-flying fast jets.

[dudbaker]

Hi

I would like to make a couple of points.  Aircraft do not all do the same thing.  A Chipmunk does not have an air data computer ADC so height information will probably come from the front altimeter.  That seat is always occupied.  Larger aircraft will used the ADC.  

QFE is used less often these days.  Airfield QNH will be used for landing with the Rad Alt giving touch down information.  It is the RAD Alt that calls 30 feet.

1013.2.  Use Flight Level
QNH.  Use Altitude
QFE.  Use Height


Dudley