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How Safe is Your Safety Altitude?

When a student pilot has filled in most of the columns on his cross-country navigation flight pad, he is then confronted with a column for SAFETY ALTITUDE although it will probably be shown as MSA. (M stands for 'Minimum').  The exhaustion of working out drift, heading, groundspeed etc may make him think, if he is flying over relatively flat parts of the United Kingdom, 'oh well, two thousand feet will do.' But it won't.

Deciding on a safety altitude requires a considerable amount of effort and thought.  It is yet another area where the professional pilot does not worry because it is all done for him.  He simply has to fly in accordance with what is put in front of him.  The overworked private pilot has no such luxury.

The chances are, if he wants to short cut working out a safety altitude, he will nominate one which is too high. ('oh well, two thousand feet will do.')  When the moment comes - the unexpected weather appears ahead - what about that two thousand feet? It gives a good margin above high ground/obstructions so why not slip below it for a bit to see how far ahead this weather extends?

This is the danger point.  Once the decision has been made to descend below nominated safety altitude there are no further limits.  Only collision with high ground/obstructions.  When descent has been made to the safety altitude - that is it.  Either maintain it or Plan B has to be activated.  Return or diversion.  No messing around & ducking below it.

Therefore the planned safety altitude has to be realistic so that it removes all temptation to ignore it.

The safety altitude has to be below the cruising altitude.  This may seem obvious, but I have actually seen an instructor's advice as 'if you are lost, climb to safety altitude and .... etc' What are you doing below your safety altitude in the first place? (And lost. But anyone can get lost).

When selecting a realistic safety altitude at the flight planning stage, there are so many factors to consider that it becomes quite a piece of work.  First of all there is the law.  Low flying, minimum altitudes over built-up areas etc.  Clearance from obstructions.  Some may be invisible. At least white wind turbines can be seen.  Masts are difficult to to see by day - they only come into their own at night when they break out in a rash of red lights.  High ground.  This would be straightforward if it were not for the wind.  Updraughts and downdraughts - particularly the latter - have been the undoing of many a light aircraft. Careful consideration has to be given as to whether the flight is planned on the upwind or lee side of high ground.  Or crossing it.  A crosscountry leg may also have to be divided with a different safety altitude after reaching a certain point.  Yet another safety altitude towards a diversion airfield has to be planned.  How far should the safety altitude cover either side of any planned track? Depends on how accurately track can be maintained and the closeness, size & nature of adjacent hazards.

Next time you are planning a flight somewhere, please give your safety altitude some respectful thought.  To misquote, safety altitude is for the guidance of wise pilots and the obedience of the risk-takers.

Adele Stephenson

Adele Stephenson instructs at Seething in Norfolk and was for many years a Captain on BAe 146s.



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