Minimum temperature for RNAV approach at Gatwick

[spikeair]

I was updating my Aerad plates with the latest this weekend and glanced at the RNAV ones for Gatwick. I noticed that there was a minimum temperature specified (-10c) for these approaches. Why's that?

Having not flown a GPS approach myself, the only reason I can think of is comparing the height / altitude being reported by GPS being compared to the QFE/QNH of the aerodrome.I'm aware that the xxx altitude on QNH XXXX on a cold day will vary to xxx altitude QNH xxxx on a very hot day , so would that be the reason?
When you fly a GPS approach, do you compare GPS derived altitudes against QNH to ensure that they are not adrift (and presumbly go around if they don't using QNH as your reference)

[chrisarrow1]

Hope this helps
http://www.pprune.org/archive/index.php/t-328419.html

[3Greens]

because at very low temperatures the altimeter will overread.

I think the formula is 8% of height for every 10deg below ISA.

[spikeair]

thanks, makes sense

[ZimmerFly]

Because it is usually "slightly" cooler in Anchorage, their RNAV charts are designed to be used down to -25C.

I don't think GPS derived altitudes are of sufficient accuracy for any approach. The altitudes/heights on any RNAV approach are referenced to pressure (or radio) altimeters. :8

[GlueBall]

I think that you can count by your fingers the number of days that the temp at London actually stayed below -10c in the past 100 years. :rolleyes:

[eckhard]

The altimeter will over-read by 4% of the height above the airfield for every 10C below ISA. Technically, the deviation from ISA should be the average figure for the air mass under the aircraft, although for an approach, the ISA deviation at the airfield should be accurate enough.

A 'minimum temperature' is established because in cold conditions, the aircraft will be physically lower at the FAF and follow a shallower approach path to MDA. This will reduce obstacle clearance. The reverse is true for hotter than standard conditions.

Hope this helps.