Emergency descend versus driftdowncurve

[Boss du Manche]

I have the following question:

We operate with a Fokker 50 over the Alps, where the MOCA is 14.000 true altitude. If during a decrompression you have to descend, my company came up with a rule to descend in 4 minutes to a pressure altitude of 13.000, to comply with the oxy-req. in the Jar-OPS. With this 4 minutes descend, you stay just above the driftdowncurve, according our flightdepartement.

My problem with this rule is that on a day with low QNH etc., you may end up 1200' below the applicable MOCA on that route, just to satisfy the 4 minutes oxy-rule and to stay above the driftdowncurve.


My question: How do other operaters fly with aircraft, not certified above 25.000', on routes where the MOCA is above the 13.000' pressure altitude.

Other question: I totally disagree with the rule that my company imposses on me, to reduce my rate of descend, so to be at 13.000 in exactly 4 minutes. (example: I fly at 23.000' and have to descend to 13.000', so my rate of descend is 2500' per minute. This means, with the power levers at flight idle, I have to increase the power again to stay at the barberpole. I am of the opinion that no matter what, you have to try to go down a.s.a.p., meaning power levers flight idle and max. rate of descend)


Please help me with a solution, because my flightdepartement is on the wrong track.

As a reference:
http://www.jaa.nl/jar/jar/jar/jar.ops.1.770.htm

(v) The oxygen supply requirements, as specified in Appendix 1, for aeroplanes not certificated to fly above 25 000 ft, may be reduced to the entire flight time between 10 000 ft and 13 000 ft cabin pressure altitudes for all required cabin crew members and for at least 10% of the passengers if, at all points along the route to be flown, the aeroplane is able to descend safely within 4 minutes to a cabin pressure altitude of 13 000 ft.

[UP2ZSKY]

I am looking at your post and in front of me I also have the Alps Driftdown Orientation Chart and see your problem. I also see much higher grid MORA's than you qoute like 18,500 near ROCCA. As for the company pushing the corner on the 4 minute rule. I don't understand why. All I can reason is that they do not want to carry the extra oxygen? I am not familiar with your aiplane but would disagree with anyone who wanted to maximize descent times in an emergency situation such as cabin decompression.
You also mentioned the driftdown curve. What is your reference to that? I do not see a relationship between driftdown and decompression. They are two very different scenarios.
I spend alot of time between 35,000 and 41,000 and just recently got into this type of discussion while crossing some very high terrain NW of YAK (Yakutat Alaska) grid MORA of 21,900.
I am sorry also if I am a little vague on Jar Ops1+ but these are new to me and I am still trying to unravel them.


[This message has been edited by UP2ZSKY (edited 25 November 2000).]

[ooizcalling]

A few points.

1. I suggest that with the disruptions from the failure that caused the emergency descent still current, that you don't really have the time to do nice little theoretical calculations to arrive at a target VSI that just satisfies the regulations 4 minute time limit. The idear is that you get your passengers down to a density altitude where they can stay alive and not having extra oxygen, 4 minutes is deemed to be the maximum time to do that without compromising their health too much....(Think of all the extra paper work if you stuff up the calculations !)

2. The high MOCA is the limiting factor in your descent options and it is 'nice' to have a comfortable margin from 'terra firma' in such a situation ...... especially if its 'stato-granitus' where you can't see the stuff but know its down there somewhere !

3. Drift down charts cover N-1 not depressurisation with a different set of criteria, ie. stay UP as long as possible (or minimise the descentgradient).... NOT get DOWN as quick as possible (well within 4 minutes anyway) but the charts do provide information on terain clearance which someone has gleaned and tried to apply to the emergency descent situation.

4. If you are approaching the most critical point of the crossing when the 'event' happens then THEORETICALY, by calculating and applying the target descent rate to meet the 4 minute requirement and doing it at barber-pole speed (conditions permitting) by adding power ... this will result in a descent gradient shallower than at throttles closed and thus the maximise clearance from terrain underneath.

5. This may or may not keep you at or above the drift down curve ... nice if it does but I dont think your going to have the time to check it out.

6. Can't you choose a route where the MOCA is below 13,000 then you don't have to worry about it. Plus, if its VMC then descent below the MOCA is possible, may be even desirable if its >13,000. Fly along the valleys at 10,000 for example.

7. But like I said in point (1) do you have time in such circumstances do do all this mental gymnastics in the middle of an emergency proceedure. I think not ! 4 minutes goes very fast in such a situation.