Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Are Flex / De Rated take offs safe?

Tech Log The very best in practical technical discussion on the web

Are Flex / De Rated take offs safe?

Old 23rd May 2008, 07:35
  #121 (permalink)  
Moderator
 
Join Date: Apr 2001
Location: various places .....
Posts: 6,519
It has been suggested that Angels60 is our erstwhile friend ssg. I don't know if that is, or is not, the case.

Can I ask that we give him the benefit of doubt for the time being and see how his posts go ... ?
john_tullamarine is offline  
Old 23rd May 2008, 18:45
  #122 (permalink)  
 
Join Date: May 2008
Location: MarysVille
Age: 59
Posts: 54
Well I think we should give him the benefit of the doubt...

Just curious with regard to this thread..doesn't Flex/assumed/derate add to take off distances, extend balanced field, and create more space for a problem between V1 and VR?

Personaly I would rather miss that crowbar on the runway at 8000 ft by hitting VR at 5000ft instead....
Angels 60 is offline  
Old 23rd May 2008, 19:16
  #123 (permalink)  
 
Join Date: Oct 2005
Location: USA
Posts: 3,218
Increased "space" between V1 and VR is not necessarily a function of reduced thrust.

If one can show adequate stopping distance from V1, it doesn't matter where on the runway it occurs. Or that mythical "crowbar" exists. However, if indeed there's a directional control problem waiting, reduced thrust provides for reduced assymetrical thrust issues and higher controllability during a rejected takeoff.

Ironically, we've had two aircraft recently experience engine fires during takeoff, with reduced thrust...both of which continued and returned to land safely. More importantly, with the reduced thrust calculation, there is no need to increase power on the "good" engine(s) throughout the entire exercise. The necessary performance, stopping or going, can all be done on the reduced thrust and still meet every margin every clearance, every gradient, every required criteria.

What do you call a runway that's five thousand feet longer than it needs to be? Long enough. What do you call a runway that's a thousand feet longer than it needs to be? Long enough. What do you call a runway that's as long as it needs to be? Long enough.

Extended engine life and reliability is more than a passing interest in safety. An engine which has been operated with reduced thrust takeoffs for it's life tends to live a lot longer, be more reliable, suffer less failures (or in other words experience longer mean times before failures--mtbf), cost less, and produce less problems.

Conversely, an engine which has been abused and pushed to it's limits regularly will have a higher probability of failure, and engines so operated have considerably reduced mean times and overhaul intervals.

Reduced thrust is safer, both in the short, and in the long run.
SNS3Guppy is offline  
Old 23rd May 2008, 19:24
  #124 (permalink)  
 
Join Date: Sep 1998
Location: wherever
Age: 50
Posts: 1,609
I'm waiting for some one to bring ATTCS into this topic. Then the fun will really start!!

There is a nice section in CS25 if anyone want's to read up first.

I'm intimate with an aeroplane where one uses reduced thrust on top of derated thrust and ATTCS.

Some of the posters on here are going to have a very hard time coming to terms with that..............................
FE Hoppy is offline  
Old 23rd May 2008, 19:45
  #125 (permalink)  
 
Join Date: May 2008
Location: MarysVille
Age: 59
Posts: 54
No doubt less engine stress leads to a probabity of more reliabity and longevity...we are getting into the areas of engineering, FOD possibities, bird strikes, puddles, ice, oil consumption issues, bearing failures, and a whole host of things that can lead to or lead away from the possibility of engine failures..

If all we ever worried about was an engine failure on take off, then I tend to think reduced thrust would further that argument quite a ways...

An AC circular I once read indicated that tire blowouts were just as likely to happen statisticaly as an engine failure.

One tire blowing on an airliner is no biggie, unless it's the Concorde I guess, on a light jet it can be catastrauphic...

V1-VR, a scary place to have a problem...if that number could be shrunk down a little, less exposure, somehow with more thrust, or changing flap positions, ect I might feel a little better trying to 'find' VR if I am committed after V1.
Angels 60 is offline  
Old 23rd May 2008, 21:56
  #126 (permalink)  
 
Join Date: Feb 2005
Location: flyover country USA
Age: 77
Posts: 4,580
V1-VR, a scary place to have a problem...if that number could be shrunk down a little, less exposure, somehow with more thrust, or changing flap positions, ect I might feel a little better trying to 'find' VR if I am committed after V1.
If that's a real issue with you (I'd like to see statistics supporting this), and if you have a range of V1's available, I suppose this could bias you to V1max.

But in the 2008 airline universe - Is it a significant issue? (I'll acknowledge that fewer tires on your bird might alter the picture)
barit1 is offline  
Old 24th May 2008, 04:00
  #127 (permalink)  
 
Join Date: May 2008
Location: MarysVille
Age: 59
Posts: 54
Well Barit, as someone mentioned, the perfectly safe flight is one that never took off.

Risk/Reward type of thing seems at play here. I have found there is always some Achilles heel in some operation. Like a main rotor in a helo coming off, or your last parchute doesn't open...that's that. I like options.

I do not mind taking calculated, mitigated risks, just hate the idea of being in position where I could've prevented something and didn't when it happened, and now it's too late...
Angels 60 is offline  
Old 24th May 2008, 19:10
  #128 (permalink)  
 
Join Date: Mar 2001
Location: I wouldn't know.
Posts: 3,954
V1-VR, a scary place to have a problem...if that number could be shrunk down a little, less exposure, somehow with more thrust, or changing flap positions, ect I might feel a little better trying to 'find' VR if I am committed after V1.
Personally i like my V1 cuts (four times a year in the simulator) to happen quite a while before Vr. Why? It is much easier to operate the airplane that way. Better controllability, perfect reference to runway centreline even in LVP, makes any kind of engine failure even more a non-issue than it allready is.
Denti is offline  
Old 24th May 2008, 20:46
  #129 (permalink)  
 
Join Date: Dec 1999
Location: Over the horizon
Posts: 230
Originally Posted by ssg
yeah I would baby those engines like you wouldn't believe..flex, reduced cruise, watch the temps like a Hawk, stay away from FOD and birds, no runups on the dirty ramps...you name it...anything to save those engines....
It is called airmanship, try it, you might like it.
Diesel8 is offline  
Old 24th May 2008, 20:49
  #130 (permalink)  
 
Join Date: Feb 2005
Location: flyover country USA
Age: 77
Posts: 4,580
Well Barit, as someone mentioned, the perfectly safe flight is one that never took off.
Yeah, but then you have to contend with terrestrial hazards.

As I said many times, there is NO SUCH THING as "safe"! Safety is a variable that can only be measured in specific units (events per 1,000,000 trips, e.g.). And on this basis, you'd be hard-pressed to beat most airlines.

(Out here in flyover country, we had a support pylon under an Interstate overpass demolished by a RR locomotive, being towed on a trailer by a long-haul trucker. What are the odds of THAT? )

BTW, my name has three syllables.
barit1 is offline  
Old 25th May 2008, 03:44
  #131 (permalink)  
 
Join Date: May 2008
Location: MarysVille
Age: 59
Posts: 54
The answer to this thread...No
Angels 60 is offline  
Old 25th May 2008, 04:08
  #132 (permalink)  
 
Join Date: Jun 2001
Location: Boldly going where no split infinitive has gone before..
Posts: 4,292
The answer to this thread...No
I guess that's why airlines who use de-rate have that terrible saftey record

You can use your excuses for caning your engines all you want- after all, you only fly for a bunch of rich guys, you don't actually have to make money out of aviation- but the stats on airline saftey re the use of derate are unassailable.

By the way- is your aircraft equipped with the absolute most powerful version of its engine, or a flat rated version? If it's the latter, YOU use de-rate on EVERY takeoff!!
Wizofoz is offline  
Old 25th May 2008, 05:07
  #133 (permalink)  
 
Join Date: May 2008
Location: MarysVille
Age: 59
Posts: 54
I have only heard one compelling argument for the use of flex...that If I was a corporate airline executive... be hoping that running those 50000 hr engines a little longer, might help increase my yearly bonus..
Angels 60 is offline  
Old 25th May 2008, 10:21
  #134 (permalink)  
 
Join Date: Oct 2005
Location: Belgium
Posts: 7
The introduction of FLEX take-offs on the AVRO-RJ fleet in my previous company (about 5 years ago) led to a fierce debate amongst pilots.

Chiefs were telling us to flex as much as possible to save mucha , we pilots could not believe that we had to increase power after "thrust reduction"...

Being an unexperienced pilot back then, and after 5 years of asking my collegues about how they operate, I finally made up my own mind.

1) FLEX does save maintenance costs
2) FLEX does burn more fuel

I made some calculations and actually, the extra fuel burn would offset the maintenance costs by 50% on the RJ. That was a few years ago with a barrel below 50 $ IIRC.

3) Performing a FLEX take-off at an average gross weight on a long runway gave us as much margin as a a REF take-off at max take-off weight on a long runway. In both cases we would not take-off in balanced field conditions, so plenty of margin.
4) Performing a FLEX take-off in balanced field conditions (short runway + average gross weight or long runway + high gross weight + high temps for isntance) is just as tricky as a REF take-off at max performance take-off weight on a medium or short runway (balanced field and/or WAT limited). You have to remind that there is no margin left, you HAVE to perform a static take-off from the calculated take-off position (including the calculated line-up position LUF, LUZ, LUH) and you better make sure every other parameter that was used for take-off is met on the runway (actual OAT on a hot runway vs the ATIS OAT, actual wind component, actual gross weight maybe different than the one you used for calculations, etc etc)

The conclusion of 4) is that in fact, because you are performing a FLEX take-off, you no dot realise that you are in the same position as a REF take-off. Performing a REF take-off, your attention is probably going to be at a higher level. Which means that you might tend to forget about the static take-off, etc.


My personal conclusion is that I used to visualize my margin. For instance, if we were to take off from full length, I would calculate from an intersection. That was going to be my margin.

I also used to increase my assumed weight by two tons (5% for an RJ) so that if I ever was in a situation were we were balanced field or WAT limited, hence a decrease in assumed temperature, we would still have that margin to play with.

Furthermore, when the assumed temp would start decreasing for the actual weight, I used to calculate a "REDU" take-off power, which in the RJ is a 10% derate from REF.


That way everybody was happy, so was I, and we always new how big our margins were in case we needed them.
flyingflatfour is offline  
Old 25th May 2008, 17:21
  #135 (permalink)  
 
Join Date: May 2008
Location: Mo
Age: 67
Posts: 8
Flex/Assumed/Derated:

Longer take off rolls =

More chance to hit birds, FOD, blow a tire
Less runway to stop in a reject
Gives pilots less reaction time to an event
More V1 -VR Spread
Further down runway, closer to obsticles
Less obsticle clearance on climb out

Fierce debate among pilots? You bet...when managment wants to put put people at risk to save a buck, not surprised that the smart pilots step up and say something...
tankdriver45 is offline  
Old 25th May 2008, 17:43
  #136 (permalink)  
 
Join Date: Sep 1999
Location: ME
Posts: 5,433
I would guess that SSG, Angels 60 and tankdriver45 are all the same person

Mutt
mutt is offline  
Old 25th May 2008, 17:53
  #137 (permalink)  
 
Join Date: May 2008
Location: Mo
Age: 67
Posts: 8
I would guess that maybe he cares about safety too...

Mutt..as to my identity...Guppy, the legend in his own mind, 135 air ambulance hack, is running around banning everyone that he doesn't like, that doesn't agree with him, or put's up a logical argument against his.

I wouldn't be here unless I thought the passengers at large might actualy benefit from these discussions, not to mention my own passengers...

Mutt, you have a brain, I can tell that..don't sell out...you want people in here that can run with you. talk the big concepts, advanced stuff...or this becomes a forum for your buddies, nothing interesting happens, no intelligent talk...no fun, nothing learned.

And if you didn't hear what I said, then no worries...I can come in here, time after time, after time, and basicaly bang the same drum...untill maybe someone listens...
tankdriver45 is offline  
Old 25th May 2008, 18:02
  #138 (permalink)  
 
Join Date: Aug 2003
Location: Sale, Australia
Age: 75
Posts: 3,829
I would guess that maybe he cares about safety too
That would be OK if he had one iota of knowledge about which he sprouts. He does get around. SNS3Guppy has his number.

You're flying a helicopter, now? You were a fixed wing captain with 10,000 hours and seven type ratings, flying a citation, and had just been rejected by Alaska airlines when you posted as ssg and then as Trickle451...both of your former identities which have recently been banned.

Now you're a helicopter pilot. Who'd have guessed?
Brian Abraham is offline  
Old 25th May 2008, 18:07
  #139 (permalink)  
 
Join Date: May 2008
Location: Mo
Age: 67
Posts: 8
Brian.. I remember kids like you..in the sandbox...you wouldn't give me back my shovel, you grew up in a troubled home, nothing I could say would sway you..

So I went over and sat on your head, pushing sand into your mouth untill you cried for your mommy..lesson learned, and like Pavlov's dog every time you picked up a shovel in the yard or a fork at dinner, you remembered the taste of sand in your mouth. It's the only thing that works..

On the flight line, if you can't figure out what a helicopter does at high speed, you don't get the job...so it's back to Microsoft Flight Sim.
tankdriver45 is offline  
Old 25th May 2008, 18:07
  #140 (permalink)  
 
Join Date: Feb 2003
Location: Blue sky
Posts: 73
95% of all takeoffs I calculate (737) are climb limited and I believe (correct me if I'm wrong) most twin engine commercial airliners are climb limited most of the time. NOT field limited so there's always extra runway to play with on the ground. You will hardly ever get a bigger V1/Vr spread, with V1 being fairly close to Vr.

The 146/Avro is 4 engined high wing, that's different but I would be surprised to see a short field performance jet being field limited on general commercial airports (not considering the exceptions of course). But I don't know the type.

Really, if you think reduced thrust takeoffs are too close to the limit, you haven't done any wet/contaminated/improved takeoff. With a wet or contaminated takeoff V1 is reduced (for a contaminated usually up to minimum control speed), and screen height at the end is reduced as well (35ft, not 50ft clearance).

Passing V1 on a reduced is relaxed, passing V1 on an improved takeoff makes you think and wonder is the aircraft is still capable to stop.

PS: improved takeoff is always full thrust by the way...
BraceBrace is offline  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.