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Are Flex / De Rated take offs safe?

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Are Flex / De Rated take offs safe?

Old 20th May 2008, 23:28
  #81 (permalink)  
ssg
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lbetter actual acceleration lift changes with acceleration=more lift with given unit time/distance-- the wings are working at a lower pressure altitude because the temp is only 'assumed' for the engines, but the engineers don't take that benefit in account and it gives a margin---

-- The engineers don't take it into account because it doesn't exist as a benefit.....so using your logic...the slower we go (flex acceleration)the more lift with a given time/distance? lol..even so...ok I will bite....you said 'benefit'..how does this BENEFIT...flex doesn't make us go faster, but slower, it doesn't reduce runway length but increases it, less initial air over the wings.=.less active control imputs and feedback along the runway untill later in the roll, less time to find out of an aerodynamic problem exists...we know Flex uses more fuel as well...so what are you saying...that we create more lift flying slower....or that flex, somewhere in the box, some computer program creats a force field of extra lifting energy at slower speeds...

And better stopping [before V1]with reverse becasue you get full rated RT [that's why it unofficial because in part 25 dry RT doesn't exist,]also spoilers and retardation devices are better in the denser air

- So flex power changes the air density around the aircraft, helping spoilers and retardation devices So flex causes a FORCE FIELD? around the aircraft to create denser air over the spoilers and retardation devices?
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Old 20th May 2008, 23:45
  #82 (permalink)  
 
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negative!!!! I mean even though you've limited the TO perf---it not as limiting as it seems because of ---let me see--- the laws of physics

think it out ----it's ok to be wrong I've been wrong many many times and I am man/ and pilot enough to admit it--and I then learned the correct answer---you've left the learning process already--yet the mountains wont move for you---your dug in deep redeem yourself now---also Old Smokey, John Tullamarine and Mutt ARE performance experts--so you should pay them for teaching you something new about performance--and I think Cessna, Mr. B and Airbus also mayknow a thing or two about performance...

edited to add that V in the 1/2 rhoV^2 ===TAS!!!!---I reread your post

Last edited by Pugilistic Animus; 21st May 2008 at 01:17.
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Old 21st May 2008, 01:51
  #83 (permalink)  
 
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SSG

Vssg is really refusal speed, a well-known concept, see also accelerate-stop.

Au contraire, my corporate jet engines (GE, RR and Honewell) are "on condition", it is a very nice selling point indicating lower costs. One of our models has flex power built into the FMS perf, btw.


BTW, after 4,000 hours in the C-5, we frequently did reduced power take-offs even at high gross weights when performance allowed. In fact, we weren't allowed not to except in exceptional circumstances. Not even to bring V1 up to Vgo. I thought it a good idea for all the reasons detailed here.

GF

You're general aviation mindset really needs to be open to life in the big leagues. Light twin ideas have no place in transport category airplanes.
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Old 21st May 2008, 02:11
  #84 (permalink)  
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Some reading on rejected take offs....


The RTO maneuver has been a fact of a pilot’s life since the beginning of aviation. Each takeoff includes the possibility of an RTO and a subsequent series of problems resulting from the actions taken during the reject. Historically, the RTO maneuver occurs approximately once each 3,000 takeoffs. Because the industry now acknowledges that many RTOs are not reported, however, the actual number may be estimated at 1 in 2,000 takeoffs. For example, an unreported RTO may occur when a takeoff is stopped very early in the takeoff roll because the flight crew hears a takeoff warning horn, stops to reset trim, then taxis back to the runway and continues takeoff.

According to these statistics, a pilot who flies primarily long-haul routes, such as in our Boeing 747 fleet, may be faced with an RTO decision only once in 20 years. In contrast, a pilot in our DC-9 short-haul fleet who makes 30 takeoffs per month may see an RTO every 7 years. Unfortunately, the pilot in each of these fleets must be prepared to make an RTO decision during every takeoff.


Boeing studies indicate that approximately 75 percent of RTOs are initiated at speeds less than 80 kt and rarely result in an accident. About 2 percent occur at speeds in excess of 120 kt. The overruns and incidents that occur invariably stem from these high-speed events.

A takeoff may be rejected for a variety of reasons, including engine failure, activation of the takeoff warning horn, direction from air traffic control (ATC), blown tires, or system warnings. In contrast, the large number of takeoffs that continue successfully with indications of airplane system problems, such as master caution lights or blown tires, are rarely reported outside the airline’s own information system. These takeoffs may result in diversions or delays, but the landings are usually uneventful. In fact, in about 55 percent of RTOs the result might have been an uneventful landing if the take-off had been continued, as stated in the Takeoff Safety Training Aid published in 1992 with the endorsement of the U.S. Federal Aviation Administration (FAA).

Some of the lessons learned from studying RTO accidents and incidents include the following:


More than half the RTO accidents and incidents reported in the past 30 years were initiated from a speed in excess of V1.
About one-third were reported as occurring on runways that were wet or contaminated with snow or ice.
Only slightly more than one-fourth of the accidents and incidents actually involved any loss of engine thrust.
Nearly one-fourth of the accidents and incidents were the result of wheel or tire failures.
Approximately 80 percent of the overrun events were potentially avoidable by following appropriate operational practices.

--------------
The Korean Air cargo jet which crashed near Stansted Airport had an engine on fire as it was taking off, eyewitnesses have told the BBC.

The jet crashed in a ball of flames in nearby fields, killing all four crew members.
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Old 21st May 2008, 02:30
  #85 (permalink)  
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.....

The Airbus was the first fully electronic automated plane, with no eletromechanic controls or cables. During their first commercial record of a take off, they lost an aircraft and a team of pilots and technicians due computer problems on board - by Captain Bill

http://www.sumo.tv/watch.php?video=3011028
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Old 21st May 2008, 02:36
  #86 (permalink)  
 
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Negative, the F-16 was first fully electronic plane in service. But the Concorde drivers here will contest that, as it had very nearly a fly by wire system. You really should learn to think and listen first.
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Old 21st May 2008, 03:36
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ssg,

what does this accident have to do with the discussion at hand. It didn't happen during takeoff but during a flyover.

Here is the cause, in case you feel like learning something:
http://aviation-safety.net/database/...?id=19880626-0
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Old 21st May 2008, 03:58
  #88 (permalink)  
 
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Nice one, Para. Except those of us in corporate aviation will have to live down the headlines. He is, unfortunately, not alone in general aviation. I had a boss who believed that it was dangerous to climb at V2+10 and the plane needed to be rapidly accelerated to climb speed. Even rotated to about 5 degrees and "let the plane fly off". Climb gradient didn't enter into his procedure, he could see the obstacles.
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Old 21st May 2008, 04:14
  #89 (permalink)  
 
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Those who feel unsafe during a flex/assumed temperature takeoff... then how do you feel like takeing off on a 35°C day?

Jet engines are "pressure" limited up to a certain temperature, up to which they are certified to give max rated thrust. This certain temperature is sometimes called flat rate temperature, or FRT. Either you use N1/EPR/IEPR values, these parameters are designed for pilots to keep the thrust at the max limit. We look up the values calculated by engineers, and use these in day to day life.

If OAT is above this FRT, the engine is limited in temperature increase. And even though we still use N1/EPR/IEPR/..., the idea behind them, the rules to calculate these values, change. That's the whole reason that makes this story possible. Assumed is not an invention or anything, it's been a part of aircraft jet engines since day 1. The higher the OAT above the FRT, the lower the thrust you are allowed to get from it. We might not see that directly, it is calculated in the N1/... values you use. In a way, you always do a kind of derated takeoff anyway above FRT (even though some might not realise this). If you would even try to get all thrust out of the engine, we are operating it outside it's limits. THAT is dangerous.

The FRT's I know for jet engines nowadays are in the 15 to 25°C region. So any day OAT is above this temperature, you are sort of derating anyway. Reading this thread I get the scary feeling some people think they always get the max thrust out of it...

So what's the difference? Assuming a temperature of 20° on a 10° day, or doing a takeof on a sunny day with 35°C if your FRT is 25°C. The first one is actually safer since real density is higher, giving slightly better "general" performance than on a 35°C day.

The "you have some safety margin extra with full thrust".... Yes, you have it that day, but if you want to fly on a 35°C day, to me, you consider that specific takeoff as being a safe thing to do that day, and the day after, and the day after,... so why can't you use assumed day in day out? It's the same thing.

Last edited by BraceBrace; 21st May 2008 at 04:25.
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Old 21st May 2008, 04:55
  #90 (permalink)  
ssg
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...............

I can't choose how hot the day is...but I can choose on cold days not to make the plane act like it's high, hot, and heavy..I can also choose on hot days to take less fuel but fly higher, up to FL450...which can mitigate the effects of even a hot day...

My risk exposure over a year and appr. 500 flights is much less the way I do it, vs the flight dept that doesnt..

End result in laymans terms: If I don't fly it to the fence on every flight then I have a better chance of not rolling it up should some failure or issue occur...
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Old 21st May 2008, 06:33
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Agh, its annoying to read something that is completely incorrect, so I’m back…..

Ssg, I suggest you educate yourself by reading FAA AC25-13 and FAA AMC 25-13. The Boeing training course presentation entitled Reduced Thrust and the Airbus “getting to grips with flex thrust”. You will then see that your statement of:

[quote] The engineers don't take it into account because it doesn't exist as a benefit.....so using your logic...the slower we go (flex acceleration) the more lift with a given time/distance?[\quote] Is incorrect. But I will stress that the comparison is between a Take Off at a specific OAT compared to Assumed OAT, it isn’t a comparison between Full Rating and Flex.
For this we are using a B777 at a weight of 235,000 kgs (That’s 518,086 lbs SSG..) S.L. Airport with 11,500 feet runway, no obstacles, Fixed Derate of 10%. That’s about average for us to do a 6 hour sector under our normal conditions.
What Pugilistic Animus was trying to point out to you is the difference between taking off at with OAT 50C, compared to OAT 25C assumed to 50C.
OAT 50C
Takeoff Distances
All Engine Takeoff Distance = 9680 FEET
All Engine Takeoff Run = 9045 FEET

One Engine Inoperative Takeoff Distance = 10055 FEET
One Engine Inoperative Takeoff Run = 8994 FEET

Accelerate-Stop Distance = 10055 FEET (1445 feet runway available)

OAT 25C assumed to 50C.
Takeoff Distances
All Engine Takeoff Distance = 9552 FEET
All Engine Takeoff Run = 8922 FEET

One Engine Inoperative Takeoff Distance = 9943 FEET
One Engine Inoperative Takeoff Run = 8897 FEET

Accelerate-Stop Distance = 9943 FEET (1557 feet runway

You can see that there is a 100 feet difference in the accelerate stop.

If we were going to use Maximum Thrust all of the time, the figures would be:
OAT 25C
Takeoff Distances

All Engine Takeoff Distance = 6298 FEET
All Engine Takeoff Run = 5783 FEET

One Engine Inoperative Takeoff Distance = 6930 FEET
One Engine Inoperative Takeoff Run = 6244 FEET

Accelerate-Stop Distance = 6930 FEET

So Accel Stop = 6,930 feet whilst with DER1/Assumed it would be 9,943 feet. It’s this difference in runway length that we are trading against reducing the takeoff thrust. But remember we still have 1,557 feet of runway left.

Even with full 10% fixed Derate and 25% Assumed Temperature Derate, we get this:
Takeoff Distances

All Engine Takeoff Distance = 10764 FEET
All Engine Takeoff Run = 10088 FEET

One Engine Inoperative Takeoff Distance = 11050 FEET
One Engine Inoperative Takeoff Run = 9857 FEET

Accelerate-Stop Distance = 11050 FEET

So even with 35% of a thrust reduction, we can still operate a 6 hour sector, and if in the event of an engine failure at VEF, we could continue the takeoff or stop with 450 feet of runway remaining.
So as you can see, we are never scraping over the fence on the way to immediate doom.
Finally to expand on what Pugilistic Animus was trying to explain to you, this comes from Boeing, I don’t know the aircraft type. It should be split into 3 columns, so it might be easier for you to paste it into excel due to formatting restrictions in PPRuNe.

Details / OAT40C / OAT15C Assumed 40C
EPR / 1.376 / 1.376
V1 (IAS/TAS) / (147/153) / (147/147)
VR (IAS/TAS) / (155/162) / (155/155)
V2 (IAS/TAS) / (162/169) / (162/162)
Thrust @ V1 /30960/31210
Thrust @ VR / 30610/30880
Thrust @ V2 / 30300/30570
So the inherent benefits that PA was taking about are shown in the speeds IAS vs TAS and the available thrust.
Using reduced thrust reduces the jet engines internal operating pressures and temperatures, which results in:[list][*] Reduced Stress and wear on the engine.[*] Reduced costs on parts and maintenance[*] Increased Engine Life[*] Increased Reliability[*] Improved operational safety and efficiency.
These words are taken straight from Mr Boeing….
That should keep you busy for a while….

Mutt (No time to edit)
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Old 21st May 2008, 06:44
  #92 (permalink)  
 
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I can also choose on hot days to take less fuel but fly higher, up to FL450
Why fly so high SSG. Its much safer flying at lower levels. What if you have an explosive decompression. By being so higher you are reducing the safety margin for a safe emergency descent. As you have said your boss would much rather be safe than save money.
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Old 21st May 2008, 07:08
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A few business aircraft cruise up to FL510, but most remain at or below FL410. Operating above those altitudes is an option for non-RVSM equipment, and aircraft wishing to avoid both weather and most of the other traffic. A lot more direct clearances above FL410.
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Old 21st May 2008, 07:27
  #94 (permalink)  
 
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Guppy,
The question was put up against the argument for safety vs cost that SSG keeps banging on about.
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Old 21st May 2008, 08:02
  #95 (permalink)  
ssg
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Mutt thanks for the numbers..

Software engineers approximations put into a fuel controller...having deviations of 100 ft over a ten thousand ft span on a 50c day..is not an 'aerodynamic advantage' ...as P.A. baited me with..

Anything other then actual OAT numbers are approximations and not some magical way of defying aerodynamic laws...

Max on your 11500 ft field is 7000 ac/stop...derate/assumed is 10000ft.
I can choose with 250 people to have 1500ft of margin after a reject or I could choose 4500 ft of margin after a reject.

Now since all the airliner guys are 'go' in here and abhor rejects, even they would have to admit that such a dangerous maneuver as a reject is better served with 4500ft margin.

--Now Mutt..you picked a 11500 ft field..you and I know that this plane is probably overgross, and the pilot is trying to squeeze that assumed 10000 ft accel/stop into a more realistic 9000 ft field with a 1000 ft stopway..

So yeah..he went right over the fence...and they do it in front of me every day...

Thanks for the numbers...
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Old 21st May 2008, 08:39
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Denti, if you and I bought our first old trashed 737 and tried to make a go of a single jet airline, our own money on the line..do or die..sure save a buck anywhere we can but be legal about it...no problem...now flash forward, billions of dollars later, hundreds of new planes on the line, we have something to lose, ..why push the safety envelope at that point....greed?
I am curious what is the WE have to pay for engines, unless you own the airline, your an employee, and management is not on yourside. You know besides mismanagement, a plane crash is pretty much the only single event that can literaly take down an airline and a company..why risk it...

The thing is, we do not produce worse safety than you do, the fact is, proven by NTSB figures, we produce 10 times less accidents than your type of operation does. I am still awaiting your figures disproving that.

And yes, private aviation often does not have the cost pressures commercial aviation has, that is in fact one of the major differences. However, even with those cost pressures we are much better at delivering a safe flight than GA aviation is, although some sectors of GA are as good as airlines in that regard, however more than enough of that sector are not.

And as to WE, well, all employees at my airline are investors in said airline, one previous big investor granted us 50% of his earnings he made with that investment as shares in our airline.
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Old 21st May 2008, 10:26
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--Now Mutt..you picked a 11500 ft field..you and I know that this plane is probably overgross, and the pilot is trying to squeeze that assumed 10000 ft accel/stop into a more realistic 9000 ft field with a 1000 ft stopway..
I picked 11500 ft coz its almost the length of the shortest runway at my local airport, the actual length is 11800 ft. So nothing devious in its selection. If the runway was shorter, the weights would be lower, and most certainly the takeoff weight would be lower. This would also probably mean that flex value would be much lower or zero.

The numbers are from the B777 Electronic Flight Manual, they are certified by the FAA and hence do not lie! So please get away from statements like these...So yeah..he went right over the fence...and they do it in front of me every day.... as you have no justification to back it up.....

Mutt
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Old 21st May 2008, 14:00
  #98 (permalink)  
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If that was a ssg 'gem', it deserves a posting holiday! How demeaning, disrespectful and insulting do you have to be in here?

JT - you up yet?
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Old 21st May 2008, 14:17
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SSG, flex/assumed takeoff is not the same as "hot & high".

Hot & high is high density altitude takeoff. Low pressure at higher altitudes in involved there as well, leading to higher required airspeeds for the same groundspeed. Not only you have lower thrust (if it's hot that high anyway), you need to accelerate to higher groundspeeds as well for the same true airspeed you need on takeoff. That's not the case for reduced thrust takeoff at low altitudes.
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Old 21st May 2008, 14:49
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SSG

Since you fly above FL 410, do you wear an O2 mask at all times? If you are single-pilot do you wear a mask at all times above FL350? In fact, if you are single-pilot are you not worried about the safety issue of single pilot? After all, the pilot is single greatest cause of accidents.

Stop banging on mx costs, airline engines routinely stay on the wing for 20,000+ hours. They are not overhauled anymore. PERIOD.
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