If I was lining up on a 7000 ft field, I would much rather lose that tire on a 4000ft take off run then on 6000 ft take of run. More time to deal with it.
Longer runs, just like longer braking distances heat up the tires more. Also longer runs tend to pick up more goodies on the runway, such as parts from other planes, the result I am sure we can all sadly remember.
Anyways... now VERY ROUGHLY AND CRUDELY...
Say a 732... My TOFL 7000ft,TOEPR 2.15 SL, 30C... 54 tons, I'm carrying 50 tons... I can derate the T/O to 47C, and it'll give me EPR of 1.96... now here's a very simplified derate effect...
54 Tons V1Bal = 143kts, Stopping distance from 143kts = 2500ft.
at 50 tons I get V1Bal = 135. From 135kts, stopping distance = 2250ft and TOFL 6500ft
AccelgoV1 dist (54tons)= 4500ft
AccelgoV1 dist (50tons)= 4250ft no derate.
Now, I derate to 40C and presto, my AccelgoV1 dist = 4500ft... stopping dist from v1 is 2250... my ASD = 6750ft, I still got 250ft of margin.
My accelgov1 dist is the same if I'm at 54tons no derate or 50tons with AOAT40C.
Now, r u trying to tell me now that because I need longer distance, I should carry 50 tons with no derate instead of 50 tons with derate, for saving 250ft of runway?
OK, how about I just load up 5 extra tons of fuel, so I use up that 250ft of runway and putting my signature "added safety measure" on the dispatcher's copy just for the sake of "oh derate is dangerous?"
Sure, I'd rather stop with 2750ft left from 135kts instead of 2500ft left from 135kts. BUT, I'd prefer 2500ft left from 135kts than 2500ft left from 143kts.
And, given the engine reaction time to go to idle from the take off thrust from 2.15 EPR and 2.05EPR, let's just assume the worst, 0.2 secs... I'm only left with 200ft added margin left! (it trundle down at 250ft/sec-ish at that speed)...
Now U want me to add the $30-50 bucks worth of extra engine wear and fuel burnt for... say, 200ft of margin, while I have an adequate 250ft of extra margin in the beginning... so the price of the extra margin in the above scenario is $0.15 per ft by not derating. Now, does the cost of changing tires due to tireburst is equivalent to 15 cents a foot?
The aircraft will do 4000 hrs and 3500 cycles this year (assuming it flies for 350 days in a year, no other downtime including for overhauls).
By not derating, you're spending an extra $100k a year to reduce your tireburst risk which on an average year will cost a fraction of that.
While insurers love added safety margins, the extra margin (to lower the premium) is offset by the fact that it's not a cost saving, therefore can increase risk in other areas (such as financial, which then leads to other operational risks), including engine failure risk. Your insurance, might actually go up!
Now, you tell me, which costs more? changing a tire or shoving the engine into the repair shop? And remember, you've already spent that extra 100,000 USD... that's 2 captain's worth of pay where I am... all that for 250ft of margin above another 250ft I already have after derate? at 4.5 to 6 crew sets per aircraft, you can pay 1 whole crew set per year with that amount of money here (including cabin crew). By derating, the company can actually afford an extra crew set, which means, up to 20% less flying hours in a year, now, that means, you can do 80hrs a month instead of 100hrs (and then find short term crew when your annual limits are up), which lowers fatigue, which means safer operations! Fatigue is a higher risk than derating or not derating! Now, therefore, with this more valuable increase in safety through cost savings from the derate, the insurers will give you a lower insurance premium...
So, now, when you do have a tireburst or an engine failure at V1, your crew can handle it better and safer! Further reducing repair costs when these mishaps happen (you can't eliminate risk! If you want to, get another job!)
hey, guess what? MORE SAFETY AND COST SAVINGS THROUGH DERATING!
So, Ssg/trinkle/whatever your next username will be, still trying to say that derating is UNSAFE?
The last tire that blew on me, was in a landing actualy,.excessive nose down, veering to the right...took all opposite control forces to maintain directional control including directional thrust management, with extremely heavy braking. Had I been a little faster on at touchdown, the nose tire would have separated from the nose gear, and what turned out to be simply changing a tire, would have resulted in an incident at the very least. I was in a turboprop...had this been a jet, with less assymetrical thrust to counter the veering, and faster speeds, this would have been an accident, as I wouldn't be able to control the direction and the gear would have impacted, metal to pavement after the tire separated..
Now would you rather have a tired crew who flies 100hrs a month going through that or a fresher crew doing 80 a month going through that experience?
Now, does your plane have 4 or more main landing gear wheels?
If you never identify it it you might not get off the ground at all. Post V1 the issue would be trying to accelerate past v1 to Vr. Same result.
A 732 had a tireburst the other week, a friend of mine was on board. It was a few knots before V1, 7000ft runway. Guess what the crew did? GO... accelerated as normal, and it was a non event except for the notable vibrations for a few secs. Had time to prepare the tire change, prepare for whatever was to be expected on arrivals... Waaaaay better than stopping, with 1 less tire (discount the brakes on that wheel), possible directional difficulty on braking, etc, etc... and causing everyone to panic and may lead to pax trying to get out without crew instructions.
Base on personal experience I don't think identifying a blown tire is difficult, it's a violent experiece...just like in a car.
Had a double blowout on 1 side two months ago... shocker, but no big deal.
Why is the recommendation from manufacturers not to abort high speed with a burst tyre. Of course if you believe the Spantax crash that killed 50 people was a resounding success then you might think it is a good idea!
I guess Trinkle/Ssg never had an airline take off briefing... it'll cover what will make an abort between 80 and V1... on planes with 4 or more main landing gear wheel...
PK-KAR