low temperature correction (or not?)
Guest
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bunnyip,
Indicated altitude above the airport is not the problem. Altimeters that are used to determine QNH are calibrated at ISA. Temperatures colder than ISA cause inherent errors.
Non-precision approaches are also not the only problem. An ILS to minimums would actually take you closer to the landing threshold and you're true altitude at DH would be lower than published if corrections for low temperature are not made.
ATC in the US will assign altitudes that are based on ISA temperatures. It is up to you to make the corrections so that your indicated altitude and true altitude are the same.
Indicated altitude above the airport is not the problem. Altimeters that are used to determine QNH are calibrated at ISA. Temperatures colder than ISA cause inherent errors.
Non-precision approaches are also not the only problem. An ILS to minimums would actually take you closer to the landing threshold and you're true altitude at DH would be lower than published if corrections for low temperature are not made.
ATC in the US will assign altitudes that are based on ISA temperatures. It is up to you to make the corrections so that your indicated altitude and true altitude are the same.
Guest
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All well and good, and great information here, but in practical terms, what should a pilot do to make these corrections and when should he do so?
On the ILS the error at 200 feet is not large. Provided the airfield has an inner marker, DME etc, and the airplane has a radio altimeter, there is no reason for the airplane to be in jeopardy. Applying the corrections for this situation is also easy.
The danger comes about in the intermediate approach phase, where the airplane is in close proximity to terrain. Under these circumstances the pilot should be aware of the errors and should make the required corrections, advising ATC of his revised altitude.
The radio altimeter will help to maintain a safe altitude and should be included in the cross reference. I have not seen this mentioned.
But in many cases the approach is not limited by terrain (eg where the airfield approach is over the sea or has no hills in the area) and therefore not applying the corrections would not put the airplane in danger.
Perhaps the procedure for applying corrections for low temperature should mention this?
On the ILS the error at 200 feet is not large. Provided the airfield has an inner marker, DME etc, and the airplane has a radio altimeter, there is no reason for the airplane to be in jeopardy. Applying the corrections for this situation is also easy.
The danger comes about in the intermediate approach phase, where the airplane is in close proximity to terrain. Under these circumstances the pilot should be aware of the errors and should make the required corrections, advising ATC of his revised altitude.
The radio altimeter will help to maintain a safe altitude and should be included in the cross reference. I have not seen this mentioned.
But in many cases the approach is not limited by terrain (eg where the airfield approach is over the sea or has no hills in the area) and therefore not applying the corrections would not put the airplane in danger.
Perhaps the procedure for applying corrections for low temperature should mention this?
Guest
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The procedure on Jepp CA-19 is quite clear. In a nutshell, the baro altimeter reads high when the temps at the reporting station are low. It IS necessary to make adjustments for this.
If flying an ATC assigned altitude, ATC will take this into account, so if you "hear" an altitude, fly it as indicated.
If you are on pilot nav, and "read" an altitude (off a Jepp chart or other printed source), then you must apply the correction (and inform ATC you are doing so). This correction may be minimal (as in an ILS DH) or many hundreds of feet (as in intermediate segments on published approaches).
Checking some MSAs in high terrain conditions will make the Cold Wx corrections critical. Your true altitude may be very close to the rocks. Even is there is no high terrain around, there are some very tall TV towers, and your radio altimeter won't help you miss those at all.
RA is a good awareness tool, but not to be relied on for these purposes.
If flying an ATC assigned altitude, ATC will take this into account, so if you "hear" an altitude, fly it as indicated.
If you are on pilot nav, and "read" an altitude (off a Jepp chart or other printed source), then you must apply the correction (and inform ATC you are doing so). This correction may be minimal (as in an ILS DH) or many hundreds of feet (as in intermediate segments on published approaches).
Checking some MSAs in high terrain conditions will make the Cold Wx corrections critical. Your true altitude may be very close to the rocks. Even is there is no high terrain around, there are some very tall TV towers, and your radio altimeter won't help you miss those at all.
RA is a good awareness tool, but not to be relied on for these purposes.
