High Terrain Escape QNE or QNH

[spierpoint jones]

As many of you are familiar with engine out escape tracks over say Andes or Himalayas etc. minimum levels are quoted . Does one fly QNH (mean sea level)or QNE.? Also ,at what point in time do you change form QNE (if that is the procedure)....

All your advise will be gratefull.

Many thanks ...

jones:ok:

[9.G]

spierpoint jones, your drift down procedure must specify the route flown with FL/ ALT prescribed respectively. However the general idea is to get you down to FL 140 where substantial time-frame is at your disposition in terms of oxygen, allowing your to decide upon further course of actions. The change to QNH is due at transition level, I'd say.

Cheers:ok:

[411A]

Does one fly QNH

Always, QNH, as that is how safety heights are presented, nearly always.

[low n' slow]

QNH is not allways readily available for your position.

Before departure, if single engine service ceiling is limiting, that is, you have terrain within 1000 feet of this level below 6000 feet or within 2000 above 6000 feet, you need a drift down procedure.

We used this procedure flying with a rather low performance aircraft in Norway, it should work much the same in larger applications...

What we did was to enter a table with our gross mass at that point along the route and our worst case scenario ice protection configuration and icing situation. Adjust the altitude given in the table for the closest QNH to convert into a FL and then adjust the point longitudinally along the route for winds.

The point is, when the s%&t hits the fan, you don't want to be messing around with finding the correct QNH for that area and then fiddling with the altimiters. With FL indicated on the altimeters, theres one less item to worry about, and the correction for this has then been done on forehand in a quiet and forgiving environment, minimizing the risk of error.

/LnS

[Bealzebub]

It is important to keep a broad mental model of the terrain you are flying over. knowing the Minimum safe altitudes is only a part of that picture if you dont also know what margins you need to factor in for the difference betwen 1013 and the area QNH or altimeter settings. It is usually a fairly easy process to find nearby (or not so nearby) airports where this information can be extracted and applied in conjuction with other airports. The QNH or altimeter setting, should give you a broad brush idea of the sort of corrections you would then need to apply to provide an appropriate safety margin. From these Met observations can often be extracted an idea of pressure gradients with a need to perhaps apply additional margins for wind speeds and temperature corrections.

It is an occaisional source of irritation when a co-pilot suggests there is little need to obtain reasonably frequent met reports on the basis that it is "a nice day". It may well be, but we still we need to know the pressure trends of airports in or adjacent to areas of significant terrain at all times when operating in such areas. A decompression is not the ideal time to be first wondering what nearby QNH/altimeter differences might be. If the information is sparce or situated over a wider area, then greater safety margins might also be prudent.