Reduced Thrust ("flex") takeoff

[Killer Shark]

Using reduced takeoff thrust, or flex, makes it far more likely that you are flying at, or near to, the aircraft's limiting performance. All to save a few engine maintenance bucks.. .Is this money saving scheme really worth the increased chances of an error becoming a safety problem?. .Personally, I would like to make my margin as big as possible, and go to full power every time.

[cisco]

Why do you bring this up?

Reduced thrust or Flex doesreduce engine wear and essentially save money.

The increase in thrust is always available and performance is not "limiting" if all the segments and requirements are met.

I would rather TO with flex and have a smaller reduction in power when Climb thrust is made.

I've never had a problem.

[Zeke]

Killer,

Another reason why you do flex takeoffs is to reduce the stress and wear on the engine making the takeoff safer, it also saves $$$ not the other way around.

Performance for single engine on a correctly calculated flex takeoff is okay, however if in your view as the PIC you require TOGA nothing stops you from selecting toga on the good engine.

Z

[Hollow Blade]

Reduced Takeoff thrust or power is used when there are no obstacles that may require the plane to use the whole amount of power available for the takeoff conditions.. .When u lose 1 eng, most planes will uptrim the operative engine allowing extra power output...also, the standard call out "max power operative engine is used".. .Keep in mind all airplanes are certified to safely climb thru all takeoff segments with one critical eng inop at max takeoff weight. If Density altitude is high, weight is close to MTOW then reduced TO thrust will not be used...for all other conditions, reduced thrust is an acceptable way of saving wear and tear of the engines.

[Le Pilot]

There's actually a greater chance of an Engine failure at full thrust.. .Anyway your Vmca is higher with a full Thrust TO.. .The times that you use Flex, derated or assumed temperature Takeoffs do save the company money and in some circumstances like Low Gross weights: Actually keeps a greater spread between your V1 and your Vmca. Safety is always a matter of opinion.

[777AV8R]

Performing a 'reduced thrust' takeoff properly is not a problem at all. In fact, in the instance that there may be an engine failure after V1 and a reduced thrust takeoff was used...may help ensure that you have a good remaining engine(s) when you really need it.

The takeoff performance is a simple calculation using a runway analysis chart for the airport. To keep from creating an error, and a good CRM technique...both pilots do the calculation separately and compare the results...if the Assumed Temp. and numbers are the same, then away you go...if not...both back in to the books again.

Bear in mind, there are several reasons that reduced thrust takeoffs cannot be made..wet/contaminated runways, special airport procedures, or MEL/CDL limitations to name a few.

[fantom]

steady.... .flex is not the same as derated. quite different rules apply. <img src="tongue.gif" border="0">

[yxcapt]

I don't have a problem using reduced thrust under the right conditions, Light loads, cool temps, long clean runways and no obsticals (above and beyound the minimum requirements).

But I do use max power more often than not. We use average weights for pax and carry on bags, operate out of tight airports (LGA for one), operate with intermixed engines, use distant and close in noise abatment departurer procedures, all this with airplanes that are 30 years old (DC-9) ect,ect. Reduced thrust has always been SCD.

There are economic consideration and benifits to reduced thrust that have been mentioned above but I beleave your have a 50/50 chance of loosing an engine on take-off, maybe you will, maybe you won't. I want to be off the runway and climb climb to a safe altitude ASAP.

As far as a reducing Vmc with reduced power that is true but you will be no where near Vmc if you fly the corrects speeds.

I comes down to judgement. I ask myself; 1. is reduced thrust authorized and 2. is it prudent for this takeoff.

[john_tullamarine]

fantom,

Just for my benefit, please ... can you indicate the rule differences of significance between flex and derate ?

. .yxcapt,

For many aircraft at light weights, low altitude/OAT, and min V2 schedules, the speed can be very close to Vmca during the takeoff rotation flare .. I have seen many a keen chap end up inverted (not to mention surprised) in the 737 during simulator exercises designed to demonstrate just this problem (and the more important concern of bank angle control under such failure circumstances). As I recall, Mr Douglas only scheduled min speed, BFL data ?

[Killer Shark]

I can help a bit with the difference between flex and a derate.

A derate rating is covered as an additional rating on the engine type datasheet. To use this rating, a full set of performance data must be included in the Flight Manual. Advantages include increased engine life, and the resulting reduced Vmc speeds are distinct advantage on some aircraft types.

Flex operations are covered by the existing engine rating and existing Flight Manual performance. The only thing that must be worked out is an acceptable method of determining the "flex" performance from the existing data, and this can be achieved in several ways, the most common (and probably most accurate without undue conservatism) is the "assumed temperature method", mentioned previously in this string.

[Zeke]

JT,

I think what he was getting at was a derate is a reduced temp that the engine manufacturer selects to give a higher TBO with a lower certified power/thrust output.

Flex or flexible temperature is the airbus term for the assumed OAT method to "fool" the engine its at a higher density height.

Z

[john_tullamarine]

That's all fine ... I was more concerned about what the rule differences were .. ie not much at all ....

once again .. my abysmal attempt at humour falls into a heap .... ho hum ...

[747FOCAL]

What does "uptrim" an engine mean? <img src="confused.gif" border="0">

[None]

I do not have a problem with flex takeoffs when they are within my comfort level. Normally there are quite a few options on the final paperwork. I like to look for a certain cushion (around 4500kg/10,000lbs) between the performance limit weight (associated with a particular flex temperature) and the actual takeoff weight. I'll ask whoever is reading the printout to look for that cushion, however there are scenarios where you are normally max'd out (BOM). In those cases one simply remains aware of such conditions.

[Killer Shark]

For the "rules", try <a href="http://www.faa.gov" target="_blank">www.faa.gov</a> and follow "Advisory Circulars". Find AC 25-13, you will find your answers. Although not really a "rule", it offers an acceptable means of compliance to allow flex and derated operations, and is widely followed in industry. This is for the US, but there is a similar European equivalent.

As for uptrim, this relates to an APR, ARP or ATTCS system (depending who certified your airplane), where the normal takeoff setting is actually a derate from the FAR 33 certified maximum takeoff power. This automatic function will "uptrim" the good engine on takeoff when the other one falls over, thus giving additional thrust. This uptrim is conventionally allowed to be up to 10% of takeoff thrust, but is not in excess of the max takeoff thrust certified for the engine. Normally only found on twins, I think that Boeing started it. Try FAR 25 Appendix I for this rule (this one IS a rule!).

[john_tullamarine]

Killer Shark,

Apologies for my previous facetious remarks .. I now note from your profile that you have a certification familiarity and/or background .. in which case I suspect that your original post was either a boring day diversion ... or a bit of a wind up ...

I have to take the view that you are being a bit pedantically engineer-ish. For the average line pilot, the distinctions between derate certification and operational reduced thrust takeoff tend to be a bit moot.

However your link to the FAA AC library ought to be noted by the guys and girls ... while the library fills up a lot of bookshelf space .. they do make for interesting and useful reading ...

[ 05 February 2002: Message edited by: john_tullamarine ]</p>

[Anti-ice]

Surely in the current crisis many are facing in aviation, take-offs at reduced power ratings (where deemed safe to do so) are a desirable pre-requisite ?

I fly on many 757/767's (on 1-2 hour sectors) with a 70% load, minimal freight,long runways ,where very high-full power settings/climb rates are used (unnecessarily) costing a huge amount of money.

These are very versatile/powerful airliners ,but is this really necessary / or just a big ego thrill for some?

Maybe some people are just into high (failure inducing) engine wear or excessive pollution ?

<img src="rolleyes.gif" border="0">

[yxcapt]

John T-

According to an old Douglas letter I have, V1 cannot be less than Vmcg, Vr will be at least 5% above Vmca and V2 will at least be 10% above Vmca. These numbers are most likely the same as the regulations. In a worst case situation, lowwest seperation of speeds was 15 knots (This was just looking at the info I have home, I don't carry a full set of performace books around).

We use the American Airlines TPS programs slightly modified for our operation. Performance (and aispeed) margines using the TPS are increased by also using an Assumed Temp (AT, This is usually well above current temp) and Assumed Take Off Weight (ATOW, at least 2000lbs above planned wt). As long as you are below AT and ATOW you will have a wider margin between Vmca and V2 regardless of the power setting (reduced or max). You will still be able to meet the required take off and departure profiles but however at slightly less margins (runway used & altitude above) than if you use actual temp and weight & max power. For those numbers you need to get out the big performance book. (FO's hate it when I do this)

I hope I wrote that clearly enough. What kind of scenario are you using to set up the Vmc roll/yaw on take-off. Seems to me the pilot would have to rotate early and/ or abrutly and then increase the pitch to reduce the airspeed well below V2 all with the engine out. In other words really get behind the airplane.

[john_tullamarine]

yxcapt,

The factors you quote are the normal factors embodied in the certification data and would appear in your DC9 AFM Section 4 (Performance). A long time since I played with a DC9 (and, to my continuing regret, I never actually flew it) .... so my recollections are quite hazy as to specific numbers.

In regard to your question on sim exercises, I have long been concerned that, for some aircraft (and the 737 is one) the operation can get back to the minimum limiting speeds under some circumstances. This is fine if the training setup exposes crews to the problems near the limits.

My concern is where the normal operator's training program doesn't expose crews to the quite critical handling problems under conditions which may be permitted or preferred by the operator's SOPs. An extreme case would be where an operation emphasises overspeed V2 takeoffs but permits the use of minimum speed schedules at light weights while only training routinely for the overspeed situation.

What I do, subject to available time, is get the crews to have a progressively more critical look at these limiting cases. It is absolutely imperative that the environment is friendly, relaxed, low pressure, capital T(raining) or P(ractice) .. otherwise the pilots get too concerned with the potential for making a "mistake".

For the Vmca concern, after a good standard has been demonstrated for routine operation failures, I set

(a) a very low AUW (with minimum fuel) for which the min speed schedule is back at, or very near to, Vmcg and 1.1Vmca

(b) SL, standard aerodrome conditions so that the thrust output is maximum

(c) and use rated thrust for takeoff

and then run the guys through the following exercises in a progressive manner so that we avoid having counterproductive inverted crashes .. although the occasional Vmca departure results which is then frozen at a suitable point. (Sometimes the stunned looks are quite interesting).

(a) arbitrarily higher than minimum speed schedule (V2 margin, say 20 kt) at max forward CG with V1 failures

(b) minimum V1/Vr/V2 schedules at max forward CG with V1 failures.

(c) repeat for the more critical failure during the rotation just as the deck cutoff angle denies the crew external visual cues and the aircraft lifts off. This maximises scan and dynamic handling difficulties. All subsequent failures are done for this condition

(d) repeat for a mid CG

(e) repeat for max aft CG

(f) if the sim has a birdstrike modelled, repeat with the birdstrike instead of a fuel cut.

(g) if we have time, after the guys are comfortable here, we do the same with a requirement to track the reciprocal localiser.

At these very low weights, the aircraft goes like the proverbial cut cat (high T/W) and by the time the failure cuts in, the actual airspeed is considerably above V2. However, the effect of bank angle on sideslip and, in turn, the "real" Vmca, is sufficiently sensitive that, if the pilot is at all slow in aggressively controlling bank excursions, the "real" Vmca rapidly increases and bites him with a resulting yaw/roll departure into a stunned frozen inverted attitude ...

After a few practice runs it all becomes a bit of a doddle with the aircraft tracking the localiser and the guys very relaxed and super-confident because they have seen and conquered something approximating the worst which might reasonably happen to them.

I can sleep well - confident that that crew isn't very likely to figure in a Vmca departure or a low level obstacle splay divergence accident. From the viewpoint of the crews (and I have had almost nil negative feedback - mostly enthusiastically positive) subsequent "ordinary" sim checkride engine failures then become a walk in the park.

The other concern is minV1/Vmcg failures with a strong crosswind. Many simulators don't model this to any extent but those which do can be used to leave the crews in no uncertain knowledge of the problems and the potential need to abandon the takeoff at a speed in excess of V1 .. that being better than having an out of control aircraft rolling itself into a ball somewhere off the side of the runway.

If we might go back to your first statement ... this sort of bare bones statement (ie not containing useful qualifications for the exceptions which can bite) in the Flight Manual or other impressively official document is the real crux of the problem. Certification requirements try to put a fence around a problem but necessarily have to approach that goal often in an artificial non-real-world way.

[ 05 February 2002: Message edited by: john_tullamarine ]</p>

[stardust]

Anti-ice,. .you state: "very high-full power settings/climb rates are used (unnecessarily) costing a huge amount of money.......or excessive pollution ?"

I don't agree for excessive pollution. The best fuel economy departure is full thrust, min flap, clean up ASAP (1000AGL) and max ROC speed at full climb thrust. Any flex settings will increase fuel burn..... and more pollution.

I fly B757 and our SOP are flex departure with flap 15/20 at V2+15..+25 up to 3000 AGL(noise procedure for all departure) and climb-2 thrust is selected at 1500AGL.. .In my view this SOP has a poor energy management (thrust reduction 1500 feet before clean up and Hi-drag reduced-thrust configuration up to 3000AGL!). . .The predicted plog fuel burn during the climb is based on a full thrust settings and 1000 AGL flap accelleration,.....we always burn 200-300 Kg extra fuel fuel during the climb!!! and they want us to go with minimum fuel.......NO

Keep it safe. .stardust

[ 05 February 2002: Message edited by: stardust ]</p>

[Croqueteer]

Some airports like LCY, a flex take off is allowed by the ops manual, but I would never consider it in the 146 as the time to recognise and act on a subtle failure is too short, and there are a great many obstacles on the climbout to the west. The occupants of Canary Wharf must be worried enough normally, but to see an aircraft staggering towards them at mid tower level would give them the s---s. The captain must make the descision regardless of the bean counters. Are you familiar with the 146, Killer shark?

[747FOCAL]

Just like to point out that there are airplanes out there that v1/vr/v2 are all about 1 knot apart. And technically, even though the AFM guys are going to throw rocks at me. V1 can equal Vr and Vr can equal V2. Nobody that I know of does that, but if are flying a Heavy run your takeoff calculations at MTOW and MAXimize Vr to clear the 35 ft obstacle. In otherwords, leave V2 what it is now and push Vr as close as possible to V2. If the MAX Vr is more than V2 set V2 equal Vr. This will cause the pilot to track down to V2 +10 instead of trying to accelerate to V2 +10. I like to call it "Flare Recovery".

[fantom]

john-tullamarine:. .slightly confused about your level of understanding here...or are you taking the mickey? you appear to have knowledge but... the basics of flex vs derate (without trying to write a thesis) are:. .flex defines a thrust level below max at which perf reqmts are satisfied.. .derate defines the max TO thrust rating LIMIT.. .broadly speaking,flex works for less than MTOW,on a half decent rwy and derate works for limiting rwys.. .If, for the same TOW, the max thrust is reduced (derated) the assoc vmcg is decreased therefore V1 is reduced; accel/stop distance is reduced and V2 can still be met.. .you knew all this already didn't you? your remark 'not much at all' was a wind-up wasn't it? <img src="tongue.gif" border="0">

[john_tullamarine]

747focal,

Certainly no problem having V1 = VR if the distance geometry fits but would be surprised to see the usual sort of swept wing jet with VR = V2 due to the significant pitch attitude change required for takeoff. I would expect that the time taken for the rotation flare would usually require VR &lt; V2 if the (normal) aim is to hit V2 at or about the screen OEI.

For prop aircraft, having a comparatively small pitch attitude change, it is not a problem to have VR at or near V2.

I take a contrary view regarding obstacles. Presuming adequate ASDA, far better to push V1 up to minimise the takeoff distance by minimising the period of acceleration (and hence distance) OEI between V1 and VR. In general, VR is chosen to suit the rotation speed increase so as to peg V2 at screen and bears a comparatively fixed relationship to V2.

fantom,

Sorry about that, mate ... yes, I have an appropriate background ... just my strange sense of humour .. I should just stick to being a boring old engineer.

As far as I am concerned, both as a pilot and ops engineer, the only difference of significance to the pilot is that, for a flex takeoff, the availability of a bit more thrust after the failure is there for the taking should the pilot have a reasonable need to do so. . .Of far more importance is adequate training of, and understanding by, the pilots within a particular operation so that they have sufficient knowledge and confidence to decide when it is, and is not, appropriate to use reduced thrust takeoff procedures (over and above the routine exceptions one normally sees in operations manuals).

The position you adopt regarding the V1/VR split is not quite so simple as just looking at Vmcg .. but we are all on the same side .. pass, friend.

[Killer Shark]

JT:. .No afternoon diversion or wind-up for me, just because I work with the stuff doesn't mean I know whether people like it or not (the use of flex, that is)!

Also, "Pedantic" is my middle name.

Croqueteer:. .I have certainly dabbled with 146 flex performance, and it's fair to say I am familiar with it. I would prefer to leave LCY by train or boat. I'm not sure whether I'd prefer to be in Canary Warf looking down at a 146, or in a 146 looking up at Canary Warf (although I'd rather be in a 146 than a Dash-8). Your call!

[yxcapt]

John T-

Interesting set up. Since I'm in the sim next month and if all parties agree, I'll see if I can set up that situation. The DC-9 is a rudder airplane when engine out and if you get the ailiron/spoilers rocking and rolling the airplane is just waving bye, bye to you.

[john_tullamarine]

Killer Shark,

Peace, brother ...

I think that the use of flex thrust has been with us for so long now that it sits reasonably well with the pilot fraternity; although I have a significant concern with the degree to which the thought process seems to have become monkey see, monkey do.

I can recall in the early days of flex ops, Wal Stack at Qantas gave his 707 pilots the carrot of a minimum 1000ft ASD pad .. seemed to quieten any dissent. I suspect that that sort of luxury probably is not the rule .... Then again, early in the piece, one crew observed that the FE pushed the throttles forward to set climb thrust .. so the procedure was amended to make the flex thrust limit not less than climb thrust.

So far as trying to maintain the best margins by using rated, rather than reduced thrust, for takeoff is, I suggest, a spurious logic. Why would we not then leave most of the payload behind ? .. that would increase the performance a whole heap more ... and as for aerodromes any higher than sea level ... and OAT above ISA - 20 .. well !!

The more important considerations are

(a) the boss needs to make a dollar or we are talking about aero club flying.

(b) the relevant limit performance cases are only reference situations ... if there is a sensible case for doing so, use rated thrust, offload some payload ... but be prepared to account for your actions at the Monday morning coffee session in the Chief Pilot's office ..

yxcapt,

Love to hear of your experiences in due course.

The nature of the normal aircraft response is that the serious problems don't really arise unless you are down in the vicinity of Vmca; then the immediate prime concern is controlling bank so that the aircraft doesn't slip into the dead engine... and, of course, it is all shake, rattle, and roll ...

One problem some of the guys experience is the new task of having to co-ordinate rudder and aileron inputs, rather than rudder with only a minor aileron balance for higher speed failures. I have found the best way to get around this problem is to emphasise

(a) control yaw-induced roll aggressively.

(b) the downgoing control horn is on the same side as the appropriate rudder

(c) have an initial guess as to the required rudder input, apply it, and lock the rudders

(d) final adjustment of control input magnitude is an iterative process .. if the rudder inputs are constrained to be step rather than a continuous modulation, then the roller coaster ride is avoided.

All good fun, the guys almost always have a ball, and the stick and rudder polish transfers quite impressively to the routine V1 failure case. Most of the guys are quite surprised at the handling sensitivity with CG ..

[ 06 February 2002: Message edited by: john_tullamarine ]</p>

[fantom]

john-t. .having re-read my note to you, it is I who apologise. don't blame me though,blame the castlemaine xxxx I was drinking.......sorry. <img src="tongue.gif" border="0">

[john_tullamarine]

fantom,

Apologies are not a concern, brother ... this is all good fun, is it not ?