Climb gradient after engine failure

[AeroTech]

Hi,

I am not familiar with these stuffs, I hope these questions don't seem stupid.

a)Are climb gradient requirements for aircraft certification or requirements that an operator should comply? or both?

b)Where these numbers (2.4%, 2.7%, 3.0%, …) came from? : Are these empirical numbers or just as a consensus or decision made by certain aviation authorities or administrations (like FAA, ICAO,…)?
Just to make it clear: for example why the climb gradient for the second segment is 2.4% and not 1.2% (twin) or for example why the second segment is usually the most limiting than the third or the fourth?

c) If a turn is required after engine failure during takeoff, does this affect aircraft performance (more/less altitude loss than usual or small/big radius than usual?) depending on the turn into the inoperative engine or into the operative engine (assume it is a twin)?

Feedback appreciated
Regards

[eckhard]

Come in JT, we need you!

[PappyJ]

a)Are climb gradient requirements for aircraft certification or requirements that an operator should comply? or both?

JAR/FAR 25.111 (or EU-OPS version)
"...The take-off and take-off flight path REGULATORY definitions...." They are regulatory for Aircraft certification. BUT, regarding the airline, when those certification criteria can not be met, the airline is required to publish a proven alternative stratagy (Ie; EOSID).

Where these numbers (2.4%, 2.7%, 3.0%, …) came from? : Are these empirical numbers or just as a consensus or decision made by certain aviation authorities or administrations (like FAA, ICAO,…)?
Just to make it clear: for example why the climb gradient for the second segment is 2.4% and not 1.2% (twin) or for example why the second segment is usually the most limiting than the third or the fourth?

Empirical? I suppose so, as they have been proved to work from time to time. However, it would be more appropriate to call them a "Decision" as they are based on regulatory requirements and determined by those agencies, with additional margins for various reasons.


c) If a turn is required after engine failure during takeoff, does this affect aircraft performance?
During a turn the aircraft is not only subjected to its weight, but also to a horizontal acceleration force. The resulting force is called "apparent weight" and its magnitude is equal to the load factor times the weight.

In short, yes it effects performance, which is why the turn is limited to 15 degrees (or another stipulated bank angle). See AMC OPS 1.495 and JAR-OPS 1.495 (or EU-OPS version)

Note that the FAA does not consider this extra vertical margin.

Does that help?

[john_tullamarine]

To add to PappyJ's post, in summary -

Are climb gradient requirements for aircraft certification or requirements that an operator should comply? or both?

The Design Standards require minimum still air, out of ground effect, climb capabilities depending on number of engines. These minimum limitations (generally referred to as WAT [Weight for Altitude and Temperature] limits) give you a reasonable probability that, in the event of a critical failure, the aircraft will continue to climb. Note that turbulence and inversions are outside of the square and may cause you to go down rather than up.

On top of the WAT limits, the operator/pilot may need to restrict TOW further to achieve other requirements, such as runway length or obstacle clearance gradients.

Where these numbers (2.4%, 2.7%, 3.0%, …) came from?

The history will be in some ancient ICAO document which I am not able to cite for you. In the nature of ICAO matters, Signatory States then implement ICAO requirements in National requirements. So, for the USA, FAR 25, for example, imposes the WAT limits on the designer for certification purposes.

why the climb gradient for the second segment is 2.4% and not 1.2% (twin)

The original Standard will be based on statistical work resulting in probabilities which meet the normal design philosophies, ie a very small probability that a failure will result in an accident. A twin will have imposed a shallower gradient than a three or four motor machine as the comparative loss of performance is much higher .. for example, considering that climb relates to T-D, loss of half the thrust may result in loss of, say, 70-80 percent of climb performance for a twin.

why the second segment is usually the most limiting than the third or the fourth?

Not necessarily the case. However, given that, once we are up and away, the main concern is terrain .. and that the terrain clearance calculations are based on net performance (gross, or reasonably expected performance less a fudge factor ... 0.8% for twins), terrain generally becomes less of a routine worry the further we are away from the runway. Also, for the majority of runways, most of the terrain problems are reasonably close in to the airport.

If a turn is required after engine failure during takeoff, does this affect aircraft performance

Certainly does. Depending on the aircraft polars the typical climb gradient decrease for a 15 degree bank turn will be something in the region of 0.5 to 0.9 percent. The general restriction to a maximum bank of 15 degrees is due to the significant ramp up in climb performance loss as the bank angle increases. Note that, for some runways, the procedure may require a smaller angle of bank to achieve a larger turn radius.

small/big radius than usual?

variation in turn radius will depend on speed so the range of V2s has to be considered in determining the turning trapezoid area to be considered for obstacle clearance.

turn into the inoperative engine or into the operative engine

not relevant unless you are going somewhat slower than you should be. Main concern with which side the failed engine is on relates to Vmca considerations where the real Vmca depends very critically on bank angle.

when those certification criteria can not be met, the airline is required to publish a proven alternative stratagy

Not the case. The WAT limits MUST ALWAYS be met in the RTOW for the runway on the day. However, if the straight ahead RTOW is not adequate for the airline's commercial desires, it is perfectly appropriate for the airline to investigate turns to see if they can achieve a better weight. A case of matching the loss in climb performance against a possibly better terrain profile and it doesn't always work .. ie sometimes the best weight is straight ahead and you just have to accept whatever the RTOW is. I can recall spending a GREAT deal of time trying to get a better weight out of Gladstone (Queensland) years ago and all I achieved was a lot of practice at drawing curved splays .. at the end of the day I had to admit defeat for the aircraft concerned and go back to the original straight ahead calculations.

[mutt]

2.4%, 2.7%, 3.0% the interesting thing is that these are only instantaneous gradients, you dont have to maintain them for the complete second segment!

Mutt

[Ocampo]

IIRC there's a segment for which (I believe for a twin) the climb gradient requirement is actually 0%. Can someone confirm o correct this?

the interesting thing is that these are only instantaneous gradients, you dont have to maintain them for the complete second segment!

Funny, I just had a mental picture of an airplane on 2nd segment pulling G's from time to time during the climb to "meet the instantaneous requirement" :}

[MarkerInbound]

First segment (35 feet till gear in the well) just has to be positive. Can be .0000001 but your net must be up.

[Old Smokey]

Who could expand upon John_T's superlative summary, just about the best precis I've seen on this topic!:ok:

Mutt speaks the truth, as always, even if the thrust is constant throughout 2nd segment, increasing TAS will steadily reduce the Climb Gradient. The technique that I use is to apply the mid Pressure Height between end of 1st Segment and Minimum Acceleration Altitude, thus, Initial Instantaneous Gradient will slightly exceed that required, Final Instantaneous Gradient will be slightly less, but the Mean Gradient should equal that required. It introduces a little conservatism between beginning and end of 2nd Segment.

To give a short answer to a question which Mutt has asked of other P/E's techniques in applying nominal higher Acceleration Altitudes (e.g. 1000 ft) than the MAA, REQUIRED Gradient is assured until MAA, and a lesser gradient accepted for further climb to the increased 'nominal' Acceleration Altitude (because it's not needed), but ensuring significantly improved vertical obstacle clearance in the 3rd segment. No significant performance penalty should result, excepting the possibility of hitting the 5 minute Takeoff thrust time limit. Most modern aircraft now have a 10 minute limit, so even this is not a real problem.

MarkerInbound, what you say is true, but the reality is significantly better than the minimum certification requirement. Typical Deltas betweem 1st and 2nd Segment Gradients are rarely in excess of 1% (0.9% is the worst that I've worked with), thus, if the aircraft can achieve a 2.4% Gross Gradient in the 2nd Segment (minimum requirement), it's reasonable to assume that 1.4% should be achievable in the 1st Segment. Even if you degrade this by the arbitrary 0.8% Gross Vs Net, we should see no worse than 0.6% in 1st Segment. Hardly rocket performance, but a darned sight better than the nominal "positive" climb gradient. I think that Galaxyflyer once stated a worse than 1% delta, but this is the exception rather than the rule.

Regards,

Old Smokey

[mutt]

OS, using AFM-DPI it appears that Boeing use 2.4% at the start of the 2nd segment, but that decreases by the end of the 2nd segment. This is different to your concept of using a MEAN value so that you were above 2.4% for the whole segment.


Mutt

[john_tullamarine]

OS, using AFM-DPI it appears that Boeing use 2.4% at the start of the 2nd segment, but that decreases by the end of the 2nd segment. This is different to your concept of using a MEAN value so that you were above 2.4% for the whole segment.

An old chestnut which has been troubling all of us for decades. In Oz, if I go back 30-40 years, the airworthiness and operational rules took opposing views. In general, the Airlines, certainly Ansett, took the view that the conservative requirement applied .. ie not less than WAT throughout the second segment.

My view has always been that, for an approved AFM for the country in which one is working, the certification is the basic authorisation unless the operational rules are more conservative, in which case the latter apply.

If one takes the view that the WAT limit really is a safety net consideration then, at the end of the day, the main concern is to have the net flight path not less than the appropriate clearance from the bumpy bits. It seems to me that a CFIT is a good recipe for really and truly spoiling one's day ...

OS' technique is a tad conservative and that is his/his airline's prerogative.

[mutt]

If one takes the view that the WAT limit really is a safety net consideration then But exactly are we achieving with the WAT limit? I am particularly talking about airliners which also provide performance limiting data that account for bumpy bits and engine time limitations.

So what OPERATIONAL use is the WAT limit?

Mutt

[john_tullamarine]

So what OPERATIONAL use is the WAT limit?

I suggest none ... other than the safety net consideration that it gives you some likelihood of not descending.

The WAT limit is a certification (rather than operational) animal in the same vein as the maximum structural limits .. just draws a line in the sand beyond which one is not supposed to go.

[Old Smokey]

Mutt,

Skipping back a few posts, yes, I'm well familiar with the Boeing Instantaneous gradient at commencement of 2nd segment, and other manufacturers cuch as McD stipulating Field pressure as the data entry point.

John_T and I were / are products of the Australian system where there was often conflict between the airworthiness requirements and operational rules were oft at loggerheads on this issue. The operational folk took the viewpoint that using a lower reference Pressure Height "consumed" a portion of the Gross Vs Net Delta as 2nd segment continued and was thus unacceptable. Consequently, reputable operators like Ansett and TAA / Australian took the more conservative approach. (I'm sure that QANTAS probably went the same way).

In practice, there's only a minor penalty, and in 99% of cases can be avoided. For a MAA of, say, 500 ft and an end of 1st Segment height of 100 ft, use of mean 2nd segment PH means using 300 ft AFL, a small penalty. This equates to approximately 10hPa conservatism, and for a B777 amounts to approximately a 2500 Kg penalty. As 99% of Takeoffs are with reduced thrust (ATM or Flex) this penalty may be recovered by a couple of degrees of Assumed Temperature reduction. In the other 1% of cases, there's a penalty, but the good folks at airworthiness at the regulatory authority won't compromise upon any degradation of a climb gradient required for obstacle clearance.

What use is the WAT limit? For my 2c worth, it offers a minimum standard of performance for the aircraft, irrespective of Runway and Obstacle factors which are another issue. Of course, we do know that there are people "out there" who go flying with WAT as the only limit regardless of Runway / Obstacle computations, but there's always a few rotten eggs in the basket.:ugh:

Regards,

Old Smokey

[Bergerie1]

So what use is the WAT limit?

I found it quite useful in The Gulf when climbing into an inversion after T/O. :ok::ok:

[john_tullamarine]

Indeed, if you are WAT-limited and then climb into a significant inversion OEI you may well find yourself dead in the water at the inversion ie if you know there is a significant inversion then it probably is a good idea to consider going into the WAT charts with the maximum lapse temperature rather than surface temperature if the former exceeds the latter.

Not a requirement but might help you not to spoil your day.

[CL300]

I will have to dig into some books, since it is a long time ago.

however, on a pure certification data collection side :

On a V1 cut takeoff, a theodolith is (was) used to measure the angle from the reference point to 400ft, a simple arithmetics would give the gradient; now, if my memory is correct in FAR25 we needed 7 valid points between 2 inflexions of any performance curve, therefore at least 7 measurements, during the life of the test program we were gathering much more data, in order to give "sensible" information, these data were crunched using statisical methods in particular the RMS methodology for Gauss distributions; leading to a sample of around 70% of the mean value for each point.

This being said, the manufacturer can publish any number within the 7 points in the Gauss distribution since he is covered by the RMS buffer; this is why some aircrafts are "flying the book" better than others.

Today, as far as I can see the gradient published in tab data is the one at 400 ft AGL, again this is the RMS gradient, leading to a conservative publication, this is why when asking the manufacturer a special set of data for specific conditions , you usually end up with performance numbers you would have not imagine.

[mutt]

Today, as far as I can see the gradient published in tab data is the one at 400 ft AGL The newer electronic AFM's show the start and end gradients for the 2nd segment, they only comply with the FAR's at the start.

we do know that there are people "out there" who go flying with WAT as the only limit regardless of Runway / Obstacle computations in the corporate world, this appears to be the norm.

The Gulf when climbing into an inversion after T/O We actually use the inversion temperature for the whole takeoff calculation. We are probably considered extremely conservative, but considering we used to have 747's DESCENDING after takeoff with all engines running, the new procedure is worth the weight loss.

Mutt

[john_tullamarine]

747's DESCENDING after takeoff with all engines running, the new procedure is worth the weight loss.

As Paul Hogan might have observed ... "now that's an inversion".

We used to see much the same result out of Tennant Creek during summer on the F27 ... wet power and next to no AEO climb.

[galaxy flyer]

OS

Yes, I did, but it applied to a certain beast that didn't meet all the civil requirements. Can you guess?

Regards to turning departures, up to a point (30 degrees) the loss of performance is kind of linear. That is, shallower bank angles lead to less loss of performance but more time getting the required turn equals, or closely equals, a higher bank angle with greater performance loss but less time spent turning. 15 degrees is the standard compromise, I believe, because most FAR 25 planes can do that bank angle at V2 without causing margin problems.

Today, departing Rifle, CO, while I had a APG Company runway analysis for CL60 departure, it was interesting to load the SID gradients and level-off heights. Loading the standard level-off height and noting the EO gradient in the FMS and then reloading the SID "top of gradient" height, it was about 15% loss in gradient going from 1500 QFE level-off to 4400 QFE level-off. Airport elevation was 5540 MSL.

Speaking of the Galaxy, departing Anderson AFB, Guam with a 600 drop to the ocean, we had ex-bomber guys who figured we didn't have to meet AFM minimum climb gradients (WAT limit) because there was nothing to hit. They were disabused of this idea, by me amongst others. True, BUFFs did sometimes disappear after lift-off (level-off?) and reappear several miles downrange.

GF

[Ocampo]

Question: when using RTOW charts how does one account for a forecast temperature inversion? If I have a relatively high flex I know I have more power available if needed but what if the conditions of the day force me to use TOGA?

IIRC, TO/ FLX is not a max. set of thrust, i.e., you can go ahead and select TO/GA if you like/need or whatever.

However, Derrated-TO is a max. thrust. Can someone with a bit more of experience in Mr. Boeing's aircrafts confirm this?

[john_tullamarine]

when using RTOW charts how does one account for a forecast temperature inversion?

One needs to look at both Hp and OAT at the inversion. Depending on the presentation of the chart and the inversion height delta, you might either correct both for pressure and temperature or run the inversion temperature back down to aerodrome level at a suitable lapse rate.

Main thing is to be aware that a significant inversion is going to present you with problems if you lose one during the takeoff so you should do something sensible to keep some extra (T-D)/W up your sleeve for the possibility. If your company procedures don't address the problem, then the above suggestions will give you a fighting chance.

Derated-TO is a max. thrust.

Doesn't matter which brand we are talking about. Flex is reduced throttle, derate is a defacto lower thrust engine bolted onto the aircraft. Flex plus derate is a combination of the two.

With flex you can run the thrust back up to the (relevant) derate or max thrust setting but, I suggest, slowly to avoid any nasty surprises.

With derate, you need to be aware of the very real potential for Vmcg/Vmca problems if you increase thrust significantly during a low speed schedule takeoff. This latter is quite a probable situation for those operators which use derate to get a better RTOW out of shorter contaminated runways - the reduced Vmcg/Vmca permits a lower speed schedule to fit better with the runway length.

[galaxy flyer]

John

Drifting into the hardy perennial--Vmcg discussion. But, this is a good one already.

GF

[mutt]

OAT plus INVERSION TEMP = Temp used for takeoff calculation.

If using FLEX/ASS then you will have lower temperature.

If using TOGA, then you will have a weight reduction........

Not many airlines use inversion temperatures due to the weight loss :)

With flex you can run the thrust back up to the (relevant) derate or max thrust setting but, I suggest, slowly to avoid any nasty surprises. ...... gotta disagree, you cannot run it back to max thrust due to lack of VMCG protection......

Mutt

[galaxy flyer]

Mutt

How about, "set thrust to the rated thrust"? In other words, from the ATM thrust to rated for the planned thrust rating, but not to a higher rating.

GF

[john_tullamarine]

Not many airlines use inversion temperatures due to the weight loss

Of course .. but, then, if you lose the engine, don't be surprised to find yourself dead in the water at the inversion ... all dressed up with nowhere to go.

... gotta disagree, you cannot run it back to max thrust due to lack of VMCG prot

You're misreading my intent, good sir.

(a) if flex only then the pilot MAY increase the thrust to the full thrust rated setting

(b) if derate without flex, no increase is permitted

(c) if derate with flex, the pilot MAY increase the thrust to the relevant derate rated thrust setting

Having seen a couple of embarrassments with thrust overswings, including one nasty fatal, any increase pemitted and availed needs to be done steadily to avoid the risk of unnecessary and, potentially, hazardous gyrations. My view is to leave the thing alone unless you are REALLY terrified of hitting the hard bits.

How about, "set thrust to the rated thrust"?

Indeed.

And, if all else fails ... and you are at rated thrust .. and still going down or are going to hit the hard bits ... it is a case of choice ... crash under control and do the best you can .. or trade a bit of extra thrust against a bit more bank as required to keep the heading under control .. and just hope that the extra sideslip doesn't bite you along the way.

One keeps in mind the reality that, on the day, the real Vmca is most likely going to be a bit lower than book so it probably isn't all doom and gloom.

If you are at a low enough weight to be in the Vmca problem region, (moreso for twin than quads) the bird probably is accelerating like a cat on a hot tin roof and, by the time it is sorted out, you will be well above the book V2 problem area.

Generally, I wouldn't be too worried about the Vmcg problem as the risk of increasing thrust is more likely to occur in the air rather than on the ground. Vmca is the one which worries me.

Main concern is in the steady state condition where the pilot has got the situation under control, is on speed .. but still going nowhere. If the operating throttle is just shoved up ... that's when he/she might find out about the sting in the tail.


Very occasionally, a crew is faced with a set of circumstances which accord with the observation that one just ought not to have got out of bed that day ... the sort of day where one earns the entire year's salary in a few minutes and either gets a pat on the back .. or a lot of tut-tuts by the Monday morning quarterbackers.

[Old Smokey]

Ocampo,

" Derrated-TO is a max. thrust. Can someone with a bit more of experience in Mr. Boeing's aircrafts confirm this?

Boeing DO indeed state that "Derrated-TO is a max. thrust", for several very good reasons -

(1) Even if you are using Reduced Thrust (Assumed Temperature Method or ATM) with a Derate, Vmcg and Vmca are based upon the full applicable Derated thrust. V1, Vr, and V2 are based upon this set of Vmc speeds. Advancing thrust to the FULL rating following engine failure at Vmc limiting speeds for a Derated Thrust Takeoff would be courting disaster.

(2) The rules specify just how much thrust reduction is possible using Reduced Thrust (ATM) as a percentage of Maximum Thrust. By introducing new sets of Maximum Thrust (e.g. TO-1, TO-2), much larger thrust reductions (as related to FULL Takeoff Thrust) are possible.

Thus, Derated TO IS a maximum thrust.

Other areas where you may find this is in Boeing recommendations such as contaminated runway operation etc., where they state that ATM shall not be used, and Maximum Thrust used, with the immediately following reminder that Derated Thrust/s are Maximum Thrust/s.

Once you are out of the Vmc danger zone (above V2 for FULL TO), you may, if operationally necessary, advance the engine/s to FULL TO with impunity, AEO or OEI. For every Takeoff that I do with Derated Thrust (which is most of them) I always check the V2 speed for FULL TO, "just in case".

John_T, you had to mention the Tennant Bl**dy Creek inversion didn't you! As a young sprog Flying Doctor pilot about 32 years ago, I had a night takeoff there which came to a stop at about 200 ft AGL. The not-so-large aircraft (8,800 Lb) just sat, and sat, and sat at 200 ft, until VERY shallow climb resumed. I've 'enjoyed' the Middle East's inversions on numerous occasions, but none came close to Tennant Bl**dy Creek's mother of all inversions. I recently saw TNK again from 38,000 feet, definately the best way to see it, still get shivers about the night that Young Smokey never made it to Old.

Regards,

Old Smokey

[john_tullamarine]

... which came to a stop at about 200 ft AGL

That's the one .. and there we was sat in our Friendly with both dog whistles screaming away at wet power ... going nowhere .. and, as I recall, we weren't at the RTOW for the day. Engine failure ? .. didn't want even to think about it.

Talk about putting the wind up two pilots. Almost enough to cause a crew to be moved to praying to the Almighty for succour. I don't recall if either of us actually got to praying .. but there certainly were a few inappropriate quasi-religious invocations uttered as we watched the bumpy bits sail past at window height ...

[Ocampo]

Thank you, Old Smokey, for quite a nice explanation. No doubts on the Derated Thrust.

I do, however, have another doubt. Here goes:

If FLEX is still a reduced set of thrust, why can you select Max. Thrust with FLEX but you can't with D-TO? Is it because Vmc speeds with FLEX are not calculated to the respective thrust reduction, but to TO/GA thrust?

BTW, some "trick" that I've learned in a few sims I've witnessed is that on the 3rd segment you are supposed to select MCT, but if you decide, you can continue on TO/GA up until the time limit is almost up, whether it is 5 or 10 mins; that way you can get a bit more of thrust in those moments you need the most. Is it good practice to do the above? Recommended or not?

Thanks in advance

[john_tullamarine]

If FLEX is still a reduced set of thrust, why can you select Max. Thrust with FLEX but you can't with D-TO?

You are still missing the difference between flex and derate - if you are at the derate setting, you ARE ALREADY at Max Thrust.

Think in the following way -

(a) for an aircraft, the certification set of rules is addressed considering the maximum power (thrust) which the engine is able to produce.

If, by some means, you operate at power output levels higher than those used for the certification then you invalidate the certification basis for the aircraft ie you are not allowed to do this.

(b) full power T/O - if you conduct a T/O using the maximum power permitted from the engine, you are conducting a full power T/O

(c) flex - you may operate the engine at a lower power setting by using less than full throttle. This is what you are doing in a flex T/O - conducting the T/O intentionally at a power setting less than the maximum that the engine is capable of producing on the day. That doesn't cause any certification problems (and, providing you have done the sums correctly, won't produce any operational problems). It follows that you will not produce any certification problems if, during the T/O you elect to return to the maximum power output envisaged by the certification basis.

(d) derated T/O - now pull the original engine out and install a modified engine of smaller power output capability. This is quite common within a family of aircraft (motorcars, etc). If you now conduct a T/O using the new full T/O power capability, you are operating to the maximum power available for this particular model engine but, compared to the higher powered version, it is a lower power output, for which the term we use is derated (ie the new max thrust rating is less than [hence derated from] the higher/highest rating. However, for the derated engine, if you are operating at max power (thrust) you are operating at full thrust FOR THAT DERATE.

(e) derated and flex T/O - exactly as at (c), you may operate the derated engine at a lower than maximum (now the derated level) power setting by using less than the throttle setting necessary to achieve the derated power output level. It follows that you will not produce any certification problems if, during the T/O you elect to return to the maximum power output envisaged by the certification basis. However, this maximum power level is now the derated level, not the original engine's higher level. If you exceed the derated level, you will invalidate the certification basis for the aircraft. Keep in mind that derate requires the OEM to schedule separate AFM data for the derate ie you have, in effect, multiple AFMs and you must use the one appropriate to your chosen level of derate on the day.

(f) with an aircraft engine, we want to be able to have our cake and eat it too - ie sometimes we want to operate to the maximum rating and, on other times, to a derated rating. However, we don't physically swap engines back and forth - that just wouldn't work in practice.

What we can do to achieve the same thing, in principle, is constrain the engine not to operate at power outputs higher than the declared reduced rating level. This can be done either by computer control or manual setting of lower power output using the engine gauges.

In doing this we are conducting a derated T/O - intentionally operating a bigger (higher thrust) engine at a lower certificated power setting. This must be done in a manner to emulate installing a lower power engine ie you must not permit the engine to operate at a power output higher than the new, lower certification rating otherwise you invalidate the certification basis for the derate setting.

Does this seem to be a round about way to achieve a goal ?

Absolutely so. Wouldn't it just be simpler to keep reducing the flex setting and not worry about the intermediate step of setting a derate setting ? Probably, but there are a couple of important things which prevent us using that simpler approach in the first instance and would deny us an advantage in the second.

(a) maximum flex setting. The rules prevent our using more than a specified maximum flex thrust reduction. See, for example, FAA AC25-13 (http://www.faa.gov/regulations_policies/advisory_circulars/index.cfm/go/document.information/documentID/22468) which amplifies the FAR 25 requirements that the actual thrust be no less than 75% of rated thrust. Given that, with some of the larger engines, it is feasible to use significantly greater flex settings than this, we need to use the "trick" of derated flex to get past the initial certification restriction.

(b) as the certification is based on the maximum power output, if we use a derate then we can get some side benefits, such as a reduction in Vmcg/Vmca. Such a benefit shows up in a reduced minimum T/O speed schedule .. which may require a lesser TOD for low weight takeoffs, allowing better payload out of shorter runways.

[Caveat - I have used power and thrust interchangeably but intend power for turboprops and thrust for turbofans]

Is it because Vmc speeds with FLEX are not calculated to the respective thrust reduction, but to TO/GA thrust?

Precisely - and the important thing is that you can have several sets of Vmcg/Vmca data, provided that the OEM choses to schedule these in the AFM.

you can continue on TO/GA up until the time limit is almost up, whether it is 5 or 10 mins

really a matter for the operator to choose for routine use.

In the event that you REALLY need the extra thrust on the day because of some out of left field consideration, I suggest that the 5/10 minute limit is not going to be very high in your list of priorities on the day. A case of whether we are talking routine or emergency conditions.

[Old Smokey]

I think that John_T's last post should be made into a sticky!!!:ok:

Mind you, in doing so, it would reduce performance related discussion in this forum to about 20%, spoil a lot of lively discussion......

Seriously, if any PPRuNers had any doubts of the how, when, where or why of Thrust Ratings, Derates, Reduced Thrust (ATM or Flex), then it's cut and paste time for J_T's post, starting with a LOT of Instructors (who are, after all, mandated to pass on such knowledge).:ok:

Regards,

Old Smokey

[Ocampo]

Thank you very much J_T, for that Condensed Performance class :ok:

Very much appreciated, sir

[galaxy flyer]

Thanks from N.A., J_T, great thread :ok: BTW, I am at the Asian Aerospace Show in Hongers, next week.

GF

[john_tullamarine]

I am at the Asian Aerospace Show

Someone further up the food chain will be there but not me, I'm afraid. I'm sure you'll have an ale on my behalf ....

[Broomstick Flier]

Very nice and informative thread, gentlemen. :D

Now one simple question, how do you learn the inversion layer temperature?

Cheers
BF

[john_tullamarine]

how do you learn the inversion layer temperature?

The met man is your friend ... normally measured using a radiosonde as, for example, in this link (http://badc.nerc.ac.uk/data/radiosonde/radhelp.html).

Probably not available for each and every airport but that would be a matter for some research into your local AIP.

[galaxy flyer]

Ask the met man for his opinion of the Skew-T chart for the location in question. It is a plot of temp and humidity over the airport. Quite a bit of information, some obscure, but will clearly reveal an inversion layer. Hope his language isn't French. :eek: Places that have strong inversions are pretty well known.

GF

[mutt]

(relevant)...... beer and pprune dont mix.... sorry JT, i should have read the complete sentence...... mea culpa....

learn the inversion layer temperature? We coordinated with the local met office to launch balloons prior to our long haul scheduled flights, plus we also accept readings from inbound crews.

Mutt

[john_tullamarine]

Some words about skew-T (http://airsnrt.jpl.nasa.gov/SkewT_info.html) charts.

[AeroTech]

Thank you for your posts and special thanks to certain gurus.

(f) with an aircraft engine, we want to be able to have our cake and eat it too - ie sometimes we want to operate to the maximum rating and, on other times, to a derated rating. However, we don't physically swap engines back and forth - that just wouldn't work in practice.

What we can do to achieve the same thing, in principle, is constrain the engine not to operate at power outputs higher than the declared reduced rating level. This can be done either by computer control or manual setting of lower power output using the engine gauges

To add few information to previous quote, here a quote from this link:Assumed Temperature Thrust Reduction (http://www.b737.org.uk/assumedtemp.htm)
When an engine is de-rated, the full (un-de-rated) thrust is no longer available because this would require changes to the EEC, HMU, fuel pump, engine ID plug and the loadable software; non of which can be done by the pilot in-flight.


a)For takeoff and approach, there is an assumption of one engine failure regarding the climb gradient. Why not such assumption on landing (3.2% with all engine operating)?

b)How a takeoff thrust time limit (5/10 minutes) can affect the aircraft (take off climb gradient, takeoff path,…)? Does this occur in reality (a fact and not only a theory) only during takeoff or even during go-around (missed approach, engine failure)?

Feedback appreciated
Regards

[Old Smokey]

Hi AeroTech,

You've asked two very good questions -

a)For takeoff and approach, there is an assumption of one engine failure regarding the climb gradient. Why not such assumption on landing (3.2% with all engine operating)?

There is such an assumption on landing. The 3.2% gradient which you've quoted is with (i) All engines at TOGA thrust, (ii) Gear DOWN, and (iii) Flaps at the landing setting. Approach Climb Gradient must be provided for, and AFM data provides for a 2.1% Gradient with (i) One Engine operative, the remainder at TOGA thrust, (ii) Gear UP, and (iii) Flaps at the Approach Climb setting. (Flaps at the Approach Climb setting are typically at or close to normal Takeoff Flap settings). There is no lower altitude limit where Approach Climb (i.e. Engine Inop) may be used, it would also apply at the DH for a Cat III landing.

PANS-OPS and TERPS provide for a minimum 2.5% Gradient during missed approach, providing 100 ft of Obstacle Clearance. It is up to you, the pilot to compensate for the 0.4% difference between AFM Approach Climb "guarantees" and the required 2.5%. (You may use either of an increased approach minima, or consult performance data to guarantee 2.5% OEI Climb Gradient, or greater if required for the location).

b)How a takeoff thrust time limit (5/10 minutes) can affect the aircraft (take off climb gradient, takeoff path,…)? Does this occur in reality (a fact and not only a theory) only during takeoff or even during go-around (missed approach, engine failure)?

Very much so, particularly for the 5 minute limited aircraft. An actual example is probably the best explanation. I was working OEI Escape Routes and obstacle data for an Australian airport with numerous obstacles in the 2nd segment climb, and a "further out" higher obstacle in the 3rd Segment. The obstacles in the 2nd Segment required a somewhat increased OEI gradient, but still provided quite useful and commercially viable RTOWs for the Runway. So far, so good.....

Although the "further out" obstacle actually subtended a quite modest gradient to the runway, and thus didn't affect 2nd Segment, it DID require 3rd Segment MAA to be increased to 1650 ft AFL. Your every day 2 engined aircraft can usually handle MAA of 1100 to 1200 ft within the 5 minute limit for Takeoff Roll, 1st Segment, 2nd Segment, and 3rd Segment acceleration to Clean configuration within 5 minutes. The (actual case) 1650 ft MAA required well in excess of 5 minutes at Takeoff thrust to reach Clean configuration. The only solution was to ARTIFICIALLY "bump up" the 2nd Segment climb gradient to somewhat more than required, in order to achieve MAA in a shorter time, and constrain the time to Vcl within 5 minutes. Thus, 2nd Segment weight limits for the higher gradient significantly reduced the RTOW for the runway, reducing these to marginally commercially viable. (A 10 minute limited aircraft would have had no problem).

So...... The time limit at Takeoff thrust can directly impact upon Limiting Takeoff Weights.:uhoh:

Regards,

Old Smokey

[ricfly744]

Hello All,

A lot is being said about temperature inversions, and as I've just started flying to the Gulf, have not seen much. Would like to know if the inversion temperature value is informed on METAR, ATIS or other means, and when this is most likely to occur. What value of temperature variation has been observed and at which hights AGL?

Thank's

Ric