Page 8 - March 2016

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aerodrome temperature for input. For airplanes not so equipped, the pilot must make the calculation manually. In either case, it’s the pilot’s responsibility to inform ATC that the approach will be flown using temperature-corrected altitudes. And while it is illegal (and unsafe) to fly an approach requiring temperature correction without making the necessary corrections, ATC is under no obligation to inform the pilot as to the transgression. Ascertaining applicability and making the proper correction is 100% a pilot responsibility.
Actually doing the calculation is relatively simple, but there are some pitfalls to be avoided. First, the correction table is based on a zero elevation relative to the Above Airport Ground Level (AAGL) height of the waypoint where the calculation is being made. So, for example, in cases where the prototypical Burlington, Vermont’s ILS/LOC Rwy 33 approach requires temperature correction, the airport elevation (335’) must be subtracted from the MSL waypoint altitude before referencing the correction table. Assuming a -30C day, typical in Vermont mid-winter, and knowing from the NOTAM that only the intermediate segment is affected:
the closest table AAGL altitude, as in this example, extrapolation for AAGL altitudes greater than 5000’ is not required. For AAGL altitudes over 5000’, the 5000’ column is used.
The Burlington case also shows that altitude corrections tend to increase in magnitude the higher the waypoint is above the aerodrome altitude. At the airport, there is, by definition, no temperature- related error, because barometric pressure and temperature sampled from the same place largely correlate with the correct altitude (as the temperature goes down, the pressure increases). But, at altitude, the temperatures are often lower than at the airport below. If they are low enough below ISA, the airport’s barometric pressure that is used to set the airplane’s altimeter and the pressure at altitude measured by the airplane’s altimeter don’t correlate well with the actual MSL altitude. If the temperatures are low enough, the error can be very large. Proof by example, the government’s formula assumes a –2C degree/1000’ normal air lapse rate. In cases where the actual lapse rate is more, the error can be even worse than the temperature correction table predicts. We see this in Vermont
AAGL
Table AAGL
Correction (ft)
Altitude to fly
Before Niduq
5500’-335’=5165’
5000’
950’
6450’ (6500’)*
Before Honib
4800’-335’=4465’
5000’
950’
5750’ (5800’)*
Before Ehiko (FAF)
3800’-335’=3465’
4000’
760’
4560’ (4600’)*
* rounded up from the table calculation for purposes of better altitude targeting in the cockpit and clearer communication to ATC.
It’s important that the pilot not make an altimeter-setting change to accomplish temperature correction. Also, although the correction value is always calculated by interpolation or rounding up (not down) from
occasionally, following the passing of a strong cold front.
The admittedly conservative Burlington example should give pause because, without correction, an airplane flying at the published minimum altitude of 4800’ MSL between Niduq and Honib could actually be as low as 3850’MSL on this particular day; 176 feet below
6 • TWIN & TURBINE
MARCH 2016