Altimeter reading temperature correction
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Altimeter reading temperature correction
Is there a more simple formula for calculating (cold weather)temperature error correction on a conventional altimeter. And lets not get into a debate about Radio Altimeters.
Question: ''On a 3'degree GS, OAT-25, OM crossing height 1760' feet, what will the altimeter indicate?'' No whizz-wheels please.
I came accross a rather complicated formula.
Correction = h(15-to)
273+to-5k(e+h)
(as fraction)
h = height above the facility
k = Standard lapse rate 0.00198вк C per foot
e = elevation
t = temperature
to= t + ke
Question: ''On a 3'degree GS, OAT-25, OM crossing height 1760' feet, what will the altimeter indicate?'' No whizz-wheels please.
I came accross a rather complicated formula.
Correction = h(15-to)
273+to-5k(e+h)
(as fraction)
h = height above the facility
k = Standard lapse rate 0.00198вк C per foot
e = elevation
t = temperature
to= t + ke
+/- 4 X ISAD X (alt/1000)
eg: for an approach in -10deg temps with an OM height of 1500 feet:
-4*-25*1.5=150ft OVERREAD.
Or do is I do, ditch the minus signs, just use 4*isad*alt/1000 and remember that colder will overread/warmer will underread.
So, to answer your question:
4*40*1.76=281
so on outermarker crossing, your altimeter should indicate 2040'.
Thank ChimbuChuckles for teaching me that one!
**Edited to correct ISAd due to the effects of no sleep**
eg: for an approach in -10deg temps with an OM height of 1500 feet:
-4*-25*1.5=150ft OVERREAD.
Or do is I do, ditch the minus signs, just use 4*isad*alt/1000 and remember that colder will overread/warmer will underread.
So, to answer your question:
4*40*1.76=281
so on outermarker crossing, your altimeter should indicate 2040'.
Thank ChimbuChuckles for teaching me that one!
**Edited to correct ISAd due to the effects of no sleep**
Last edited by compressor stall; 16th Aug 2003 at 04:52.
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I thought everybody used the recommendations in New Pans OPS? Add 4% for every 10deg. below ISA.
The important thing is to keep it simple. You don't have time to mentally calculate this when briefing/flying the approach.
In your case, it would be ISA-40.
And I don't mean ISA-37, because using the most conservative value (METAR OAT, GND level) is good airmanship. You don't know if there's an inversion above the airfield, and the most important altitude to be corrected is the DA/MDA, right? I'd also use whole 5%-corrections to keep it simple.
So, an OAT of ISA-40 would give a 16% increase in altimeter reading (use 15% or 20% when calculating, and round the answer up/down). The altimeter error would be added to your height above the aerodrome, e.g. if the aerodrome in this case were at 500', you'd have to add ~16% to 1260'. Your altimeter would read about 2050-2100' when passing the marker. Remember; the marker signal is a cone (area) rather than a fixed point, and your altimeter isn't that exact, so a 100% correct reading is seldom the case, even in ISA conditions.
Suggest you read New Pans OPS; there's even a table of altitude corrections you can carry in your RM or Pinto Board.
I started my flying training in Northern Norway, where the OAT drops to, eh, well, I'd rather not think about it. We used this rule of thumb from day one and it's still what we use in my airline when the OAT drops below ISA.
The important thing is to keep it simple. You don't have time to mentally calculate this when briefing/flying the approach.
In your case, it would be ISA-40.
And I don't mean ISA-37, because using the most conservative value (METAR OAT, GND level) is good airmanship. You don't know if there's an inversion above the airfield, and the most important altitude to be corrected is the DA/MDA, right? I'd also use whole 5%-corrections to keep it simple.
So, an OAT of ISA-40 would give a 16% increase in altimeter reading (use 15% or 20% when calculating, and round the answer up/down). The altimeter error would be added to your height above the aerodrome, e.g. if the aerodrome in this case were at 500', you'd have to add ~16% to 1260'. Your altimeter would read about 2050-2100' when passing the marker. Remember; the marker signal is a cone (area) rather than a fixed point, and your altimeter isn't that exact, so a 100% correct reading is seldom the case, even in ISA conditions.
Suggest you read New Pans OPS; there's even a table of altitude corrections you can carry in your RM or Pinto Board.
I started my flying training in Northern Norway, where the OAT drops to, eh, well, I'd rather not think about it. We used this rule of thumb from day one and it's still what we use in my airline when the OAT drops below ISA.
