Punk666 and others,
During my JAA IR I was told to add 50' on top of the DA for an ILS which I never understood.
Yeah I'm pretty certain we taught the same but I really can't remember the reason behind it. I remember it made perfect sense though when I was told....Bollocks this is going to do my head in now till I find out!!!
The only guess I could make (which I don't think is the real reason for it) is that I think the limitations were +100 -0 so maybe adding 50ft to DA was a safety buffer so the student didn't go below DA before making a decision?...on second thoughts that can't be the reason!!
Anyone able to help?
I think I can answer this one. I spent about 1000hrs training guys and gals for the CAA initial IR test. The criteria for the test required (amongst other things) that the candidate initiate GA from DH -0' +50'.
We used to recommend that candidates THINK about commencing the GA at DH +50' to ensure that they didn't COMMENCE it below DH. Of course, it was (and still is) acceptable to dip slightly below the DH during the GA manoeuvre, provided that the GA was commenced between DH and DH +50'.
PEC had nothing to do with it, as this was a value which was tabulated in the AFM and could be applied if necessary. From memory, the light twins that we used had negligable PEC at SL airfields and approach config/speeds.
For the NPA, the candidate had to level off at MDH and then maintain MDH -0' +100'. For this reason, we recommended adding 50' to the MDH and using this as the 'target'. This gave the candidate + or - 50' to play with while he flew towards the MAPt, tracking the NDB (which by now was behind him/her) on the RBI and working out how long to run to the threshold from the stopwatch which he/she had forgotten to start.
If in doubt, when the examiner asked' 'how long to run?', we used to say, 'just tell 'em 40 seconds. It'll be about right'.
All of this was in the late 1970s and early 1980s, using hand flying (without FD) and using QFE for the approaches.
Also, we used 'dive and drive' for the NPA.
Happy days!
Eck