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As a rookie pilot trainee I'm confused about the folllowing statement in the manual (page 25) of my Diamond DA40 training A/C

"QNH: Theoretical atmospheric pressure at MSL, calculated from the elevation of the measuring point above MSL and the actual atmospheric pressure at the measuring point.

Indicated Pressure Altitude: Altitude reading with altimeter set to 1013.25 hPa (29.92 inHg).

Pressure Altitude: Altitude above MSL, indicated by a barometric altimeter which is set to 1013.25 hPa (29.92 inHg). The Pressure Altitude is the Indicated Pressure Altitude corrected for installation and instrument errors. "

This seems to ontradict my earlier understanding as well as other sources thar QSH and Pressure Atltitude are referenced to two different datums. The DA40 manual suggests that MSL and 1013.25hPa refer to the same datum.

Can someone help me out here? Thanks

QNH = Pressure Altitude only when the (theoretical) sea level pressure is in fact 1013.25. In reality, the sea level pressure on the day is different.
E.g. the current METAR for Heathrow shows Q1030, i.e. substantially higher, the equivalent of roughly 500 feet... If you kept your altimeter on 1013.25 it would show - 500 feet when you had landed (and it would have shown 0 when you were still at 500 feet) . Probably safer than the other way around, but incorrect altimeter settings can lead to bad surprises!

Referencing the QNH is necessary when you are close to the ground or underneath low level controlled airspace, or indeed as you are crossing controlled airspace.
Referencing 1013.25 (and thereby expressing your altitude in Flight Levels) is useful when you are at high altitudes (above the transition altitude/level/layer) and in the airways system to separate aircraft vertically, who all use the same pressure reference.
(Referencing QFE is only useful - if at all - when you are in the circuit of the aerodrome the QFE relates to).

Morten

That is or better was my understanding as well. Maybe it is just me but the DA40 manual seems to contradict this. Link on page 25

I must have misunderstood your question...

You write:

The DA40 manual suggests that MSL and 1013.25hPa refer to the same datum.

I don't see that in your quote.
QNH is referred to the pressure at MSL.
Pressure altitude is referred to 1013.25hPa

If (but that's a big if) we have an 'ISA' day, then it so happens, by definition, that pressure at MSL = 1013.25hPa.
But that rarely happens.
Which you say is how you understand it.

So what's the question ?

Morten

My question is. Why does the official DA40 pilot handbook state:

"Pressure Altitude: Altitude above MSL, indicated by a barometric altimeter which is set to 1013.25 hPa (29.92 inHg). "

To me this seems to contradict your (and my) interpretation.

LOL.

I really think it's just the way you're reading it.
Try this:
"Pressure Altitude: Altitude above [a hypothetical] MSL, [when the hypothetical MSL is] indicated by a barometric altimeter which is set to 1013.25 hPa (29.92 inHg). "
Or
"Pressure Altitude: Altitude above [a datum (let's call it MSL just for fun, but we could call it anything)] indicated by a barometric altimeter which is set to 1013.25 hPa (29.92 inHg). "

Whereas the datum for both is MSL, the 'MSL' used for Pressure Altitude is not the real or actual MSL but the ISA MSL.

?

LOL

Thanks. Didn't know that pilot handbooks required those levels of imagination. Always thought of flying as a clear rules, no margin for ambiguities industry.

I'm afraid I disagree with Morten. It is fundamentally misleading to insert the term 'MSL'.

As defined by the FAA*:

'Pressure altitude is the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.92 "Hg (1,013.2 mb) as measured by a barometer'

*other definitions from other more geographically relevant jurisdictions (but nevertheless consistent) are available,

Always thought of flying as a clear rules, no margin for ambiguities industry.

You really are a rookie trainee pilot aren't you

Was.... I lost my innocence a few post ago

Sorry to have burst the bubble...

Properties of a 20th century GA pilot: steely-eyed, square-jawed, cat-like reflexes, mammoth-like strength... and 20 years experience reading between the lines.

I agree with @GrahamB - the use of the term 'MSL' is confusing at best and misleading at worst. But you'll unfortunately have to get used to it .

When you sit your exams read every question carefully. Twice. Then backwards word for word and then twice again.

Morten

GrahamB wrote:I'm afraid I disagree with Morten. It is fundamentally misleading to insert the term 'MSL'.

As defined by the FAA*:

'Pressure altitude is the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.92 "Hg (1,013.2 mb) as measured by a barometer'

*other definitions from other more geographically relevant jurisdictions (but nevertheless consistent) are available,

And on a day with a ISA that SDP will be coincidental with MSL.
(is this the bit that is causing confusion?)

I don’t buy it. It’s a cock-up, plain and simple.

They probably mention a barometric altimeter as most aircraft use them, rather than a GPS reading or a radar altimeter.
As for ambiguity, that's what aviation is all about, not removing it, that would definitely make things far too easy.
Heights in feet, horizontal visibility/runway lengths in metres, distances in nautical miles.
Airspeed in knots, miles per hour, kilometres per hour depending on who built the aircraft.
Fuel in litres, gallons, pounds or Kilogrammes.
Altimeter settings in QFE, QNH, RPS, ISA.
True tracks off the chart, magnetic compass headings to steer by.
No ambiguity whatsoever