Hi Mutt,
Busy couple of days on the keyboard huh?? Enjoy the leave mate.
For UK operations I can't offhand think of anyone who doesn't derate as far as possible unless prevented by individual runway, weight (really must remember to say mass....), obstacle, MEL or contamination problems.
Crews understand the reasons for it and take it seriously. Most outfits use tables to completely fine tune the power setting chosen rather than just taking fixed stages of derate.
As yet no-one is using laptop devices for calculations in the UK a la Southwest or Fedex but the CAA are keen to see it come about. It's one of the main projects I've beeen working on for the past year. The technical issues are zero but data, its processing and it's validation to UK and JAR standards rather than FAA has been problematic, especially regarding your favourite topic of contamination
(Mutt has been trying for 18 months that I know of to get some decent guidance on slush/snow/ice) Luckily, colleagues at Braathens have led the way and very kindly allowed me to hang on to their coat-tails while they do it - including the writing of acceptable contaminated performance software.
Meanwhile, the tabular formats are the system generally in use and familiar to most of us. They vary in terms of how far they go up in assumed temperatures. Some operators go as high as the top end you mention while others stop at the aircraft ambient temp. limit. Where I work they only go up to 48 degrees. An entirely arbitrary but pragmatic limit to fit comfortably on the page and be transmitted usably by the lowest common denominator downroute - fax
General rule for UK ops is a fixed maximum derate of any given engine type. The derate chosen on the day can never be less than this
or the climb power selected. This includes derated climb settings.
Just to keep it crystal clear: I derate at every possible opportunity. I take the time to talk to the engineers regarding their trend monitoring work (full QAR fit) and have a great deal of faith in their efforts and experience.
However, it was seeing examples of performance calculations using every inch of runway soley for maximum derating and no other reason that prompted me to write. There is very clear evidence of the benefits when derating operational engines - it's not theory. In my experience though, powerplant experts seem to shuffle and shrug a lot when discussing the availability of data supporting the advantages of really deep derating.
I'd honestly suggest you consider talking this over with a QA person. Risk analysis is something they can give you guidelines on. Changes in performance rules are glacial but change they do - regretably the final push is usually induced by serious accidents.
The one second Vef rule you mention is entirely due to this. Also, the part worn brakes item was fought for over many years but only came about after the total loss of a wide body aircraft. I know it must seem like ancient history to you but all performance calculations for the first 20+ years of jet transports were based on new brakes, tyres and a scrubbed runway. It seems laughable and illogical now but I promise that what you're working with today will seem just as daft and dangerous when you look back in a few years - holding a pint and your active ATPL.....
Yes, a pad is entirely arbitrary but, as John T so lucidly points out, in a litigious world even the most elementary models of risk analysis will point to the inclusion of a buffer, whether fixed or percentage based, rather than a few degrees more aggressive derate.
I think this exchange of views across disciplines is incredibly valuable - makes the aggro associated with running this place seem worthwhile. My sincere thanks to all the tech log regulars.
Regards from the Towers
Rob Lloyd
[email protected]