Yet another takeoff segments thread

[Coolingthunder1]

Hi all,

So I have been brushing off the ol' class A performance stuff, and have managed to confuse myself.

So, second segment is 2,4% for a twin, which is the gross gradient.

If somebody were to ask you: What is the minimum NET gradient a twin engined aircraft needs to achieve in the second segment? In this case, is it 1.6%? (2.4-0.8 correction)

And so, for obstacle clearance considerations, the NET flight path has to clear obstacles by 35ft, does this mean we assume a slope of 1.6%, which clears all obstacles by 35ft?

[Lantirn]

Your statements are all correct. Yes the net is 1.6% on 2 engine a/c and yes it clears obstacles by 35ft

Yet many confuse the net with the gross

Gross is what the a/c will fly actual on one engine and net is the penalty where the obstacle limitations apply for the 35ft clearance

That means that the actual path of the aircraft (gross) will diverge by 0.8% for all the takeoff profile by the obstacles (in opposition with ppl which believe that the actual a/c will have 35ft clearance only for all the takeoff profile)

Also an obstacle limited takeoff (penetration of -0.8% and failure to maintain adequate clearance) will have to be mitigated with improved climb by the software which will generate increased V2 to achieve the required increased gross gradient which obviously will be more than 2,4%

[john_tullamarine]

A few comments ...

First, there are several different climb concerns:

(a) the certification WAT limit climbs which are interested in height and temperature, only. If, at MTOW, you can't achieve a particular WAT requirement, then you need to reduce below MTOW to do so. WAT limits are line-in-the-sand minimum gradients to give you a reasonable chance of getting away from the runway and other hard and rocky bits as well as defeating the problem of aircraft which only successfully takeoff due to the Earth's curvature.. They have naught to do with obstacles or other operational considerations.

(b) if, for the runway on the day, at either MTOW or a lesser WAT limited weight, you still can't clear one or more obstacles by the required margin, then you have to reduce weight further in order to do so (or use some other acceptable technique, such as overspeed V2). Nothing to do with the WAT limit, solely a matter of not getting the net flight path too close to the obstacles. While folks carry on with gross and net, we really have little interest in gross - it's mainly to do with net.

(c) operational climb requirements, such as SIDs or other ATC directed requirements.

If somebody were to ask you: What is the minimum NET gradient a twin engined aircraft needs to achieve in the second segment? In this case, is it 1.6%? (2.4-0.8 correction)

So long as you keep in mind that this is a WAT requirement and has nothing at all much to do with obstacle clearance.

And so, for obstacle clearance considerations, the NET flight path has to clear obstacles by 35ft, does this mean we assume a slope of 1.6%, which clears all obstacles by 35ft?

First, the 1.6% is not concerned with any obstacles, it is SOLELY a WAT (certification) consideration. For obstacles, you run the standard net flight path analysis and adjust TOW, as you might require, to achieve the required NFP clearance from obstacles. Generally, the mishap-risk concern is mainly with closer in obstacles as, assuming the aircraft is operated sensibly, the gross/net decrement will put the aircraft well above the NFP the further out we get from the runway. This often confuses the line pilot who, knowing that the critical obstacle is, say, that hill at 3 miles, then has difficulty reconciling the observation that, at the maximum RTOW with a V1 failure (ie in training) the obstacle is way below the aircraft. How come ? wasn't the clearance 35' ? Yes, but the 35' is tied up with NFP but you might be a moderate way above GFP, depending on how you are operating the aircraft.

Your statements are all correct. Yes the net is 1.6% on 2 engine a/c and yes it clears obstacles by 35ft

No, quite incorrect. First the 1.6% is OEI, NOT AEO as might be inferred from your comment. The 1.6% is a WAT limit and knows naught about obstacles. The NFP obstacle analysis, which will keep the RTOW at, or below, the WAT limit is the thing which looks after the obstacle clearance.

Gross is what the a/c will fly actual on one engine and net is the penalty where the obstacle limitations apply for the 35ft clearance

Actual will generally be a little bit better than GFP.. The WAT net penalty has NOTHING to do with obstacle limitiations.

... will have to be mitigated with improved climb ...

That's one technique we use but not the only one. overspeed V2 (improved climb) is of little use for close-in obstacles due to the longer OEI TOD needed to achieve the higher V2.


Having gone back to classroom teaching of pilot theory subjects in recent years, I am regularly amazed at some of the wrong ideas coming up from the lower licence grades. I can only think that there is much to be desired in the quality and knowledge of the instructors along the way. Then, again, I can recall from my own airline flying having more than a few checkies pontificate in substantial ignorance on this and that topic .....

However, stick with the Perf "A". Providing you get some decent instructors, you will pick up a lot of useful gen which folks in other jurisdictions might not.

[Lantirn]

For obstacles, you run the standard net flight path analysis and adjust TOW


What is the gradient of the standard net flight path analysis assuming no obstacles?

Also I wrote "will diverge by 0.8% for all the takeoff profile by the obstacles" obviously meant minimmum, by the net.



Your statements are all correct. Yes the net is 1.6% on 2 engine a/c and yes it clears obstacles by 35ft

No, quite incorrect. First the 1.6% is OEI, NOT AEO as might be inferred from your comment. The 1.6% is a WAT limit and knows naught about obstacles. The NFP obstacle analysis, which will keep the RTOW at, or below, the WAT limit is the thing which looks after the obstacle clearance.



I thought takeoff segments were only for N-1 engines. Thats why I wrote that for 2 eng a/c, it is 1.6%.

I am not a perf engineer neither a perf instructor, but anyway these are always interesting topics that I like to learn from well educated guys

(c) operational climb requirements, such as SIDs or other ATC directed requirements.

Sorry to kindly ask, but what these have to do with single engine performance? SIDs and ATC requirements are for all engines ops.

[Alex Whittingham]

The relevance of the confusion about minimum gradients is that the Climb (WAT) limited gradient is an air gradient and obstacle clearance gradients are related to the ground, ie influenced by wind. There is a huge amount of confusion caused by fuzzy thinking at ATPL level.

[Lantirn]

The relevance of the confusion about minimum gradients is that the Climb (WAT) limited gradient is an air gradient and obstacle clearance gradients are related to the ground, ie influenced by wind. There is a huge amount of confusion caused by fuzzy thinking at ATPL level.

I got through that and it was not an easy matter to understand them well although they were a favourite subject of mine.

After some years of jet flying and using computerised EFB performance, I have sometimes to really dig in the books again to clean up things on my head, mostly due to the fact that no one is talking about performance these days. You get a number, we are good to go. The ideas that ppl sometimes discuss in the cockpit are non sense mostly because performance these days is teached but very rarely properly applied and discussed in the cockpit unless there is an obvious reason. But a/c these days are so overpowered that make ppl stop thinking about it

[john_tullamarine]

What is the gradient of the standard net flight path analysis assuming no obstacles?

There is no NFP analysis if there be no obstacles. Either you are MTOW or WAT limited (ignoring cruise and landing considerations which may further limit TOW).

Also I wrote "will diverge by 0.8% for all the takeoff profile by the obstacles" obviously meant minimmum, by the net.

That's fine, I don't think we have any disagreement there.

I thought takeoff segments were only for N-1 engines. Thats why I wrote that for 2 eng a/c, it is 1.6%.

Provided the aircraft is operated such that the GFP remains above the NFP, all is fine. Again, we don't have a disagreement on this point. However, again, you appear to be overlooking the distinction that 1.6% is a minimum WAT requirement and, apart from being that minimum available gradient, has naught to do with obstacle NFP analyses.

I am not a perf engineer neither a perf instructor,

Which is why I like to see robust tech discussions in this Forum. The new chums coming up through the ranks get to see more than perhaps what they were exposed to during their initial training and they get the benefit of line observation feedback from you guys who are out there trying your best not to get killed. Unfortunately, unless the performance instructor has a sound ops eng background, the instruction often becomes a bit tenuous. There are non-engineering pilots about who do have a very good background but they are in the minority. One who comes to mind and posts on PPRuNe is Centaurus.

what these have to do with single engine performance? SIDs and ATC requirements are for all engines ops.

Nothing, unless you find yourself with a quiet engine during the procedure. I introduced these simply to list a variety of climb considerations which the Commander should be taking into consideration in his/her overall decision making processes.

The relevance of the confusion about minimum gradients is that the Climb (WAT) limited gradient is an air gradient and obstacle clearance gradients are related to the ground, ie influenced by wind.

And, if I may emphasise, WAT is for nil wind, OGE.

There is a huge amount of confusion caused by fuzzy thinking at ATPL level

Oh, indeed, good sir, which is why this forum is so important on the site for the educational imperative. Competent instructors, such as yourself, are, unfortunately, fighting a rearguard action against the rest. I despair at the training organisations who use their flying instructors to teach theory when many of those instructors barely comprehend the subject details themselves.

mostly due to the fact that no one is talking about performance these days.

Which is why this forum is so important. We have a good cadre of very expert tech people on PPRuNe - if you were to consult privately, you would pay through the nose for what you get free in this Forum.

You get a number, we are good to go.

Most of the time ...

But a/c these days are so overpowered that make ppl stop thinking about it

That just means the weights are pushed up. Where a major problem arises is, especially for twins, we see tremendous performances routinely AEO ... however, OEI and seriously weight-limited and it's a very different story.

[Lantirn]

What is the gradient of the standard net flight path analysis assuming no obstacles?

There is no NFP analysis if there be no obstacles. Either you are MTOW or WAT limited (ignoring cruise and landing considerations which may further limit TOW).

That’s where 1.6% applies if I get it correctly.

I thought takeoff segments were only for N-1 engines. Thats why I wrote that for 2 eng a/c, it is 1.6%.

Provided the aircraft is operated such that the GFP remains above the NFP, all is fine. Again, we don't have a disagreement on this point. However, again, you appear to be overlooking the distinction that 1.6% is a minimum WAT requirement and, apart from being that minimum available gradient, has naught to do with obstacle NFP analyses.

I am not overlooking that, that’s why I wrote that anything protruding the net has to be accounted so the gross will always have a minimum 0.8% separation from the net. Maybe I still don’t get something here but in simple words I understand is that if an obstacle protrudes the net, then the net is elevated. Add 0.8% to that and you have the new gross gradient

[john_tullamarine]

That’s where 1.6% applies if I get it correctly.

You appear still to be missing the point that the WAT limit has nothing to do with obstacles and, vice versa, other than to the extent that, when running the NFP analysis, the floor is the WAT limit gradient for the particular segment being considered. If there are no obstacles of concern then, for the second segment (in which you appear to be interested) you will be limited either to the certificated MTOW (for which the gradient may well be greater than the WAT limit) or, if the MTOW gradient starts to compromise the 1.6% second segment WAT requirement, then you start to reduce weight to maintain not less than the WAT gradient requirement.

if an obstacle protrudes the net, then the net is elevated. Add 0.8% to that and you have the new gross gradient

Not quite. It is a mix and match between playing with weights and speeds to achieve the required net clearances. If, at your first guess weight, you aren't going to achieve the NFP clearance then you either reduce the weight and/or play with overspeed or use a curved departure. The NFP surface will increase in elevation (at that point) to get to the required clearance but the gradient isn't going to remain the same. Apart from the observation that knowing the gross gradient is useful to give you an idea of a minimum target ROC, gross gradient really is of little, if any, interest to us at all.

[Lantirn]

Sorry, it’s my mistake the way I wrote things. English is not my native language and it seems that my wording can be understood differently.

Ofc no obstacles means respect the climb limit. If there are obstacles, then respect the obstacles.
I meant elevated in terms of gradient, not in terms of altitude.

And yep, this applies for all the segments not only the second segment.

By the way: What is a curved departure? First time I see that.

[john_tullamarine]

Sorry, it’s my mistake the way I wrote things.

First, my sincerest apologies, good sir, I didn't realise that English was not your native tongue. Absolutely not your mistake but, rather, mine.

What is a curved departure? First time I see that.

Normally, we takeoff and maintain runway track to a suitable elevation and then turn to pick up the outbound course. If, however, a straight flight path departure is too restrictive for TOW, often we can find a curved, or turning takeoff departure, which gives a better RTOW and payload. Such turns can commence from very early in the departure, often at runway head, or at some specific position or altitude. There are various things which we take into account when designing such departures but, for the crew, the departure will be defined in the ops manual and the crew just follow what the book says for the runway. Flying a curved departure, though, is quite demanding as location and tracking is critical. As a consequence, speed and bank angle control is critical to keep the aircraft fairly close to the presumed tracking plan so that we don't get closer than intended to the obstacles of concern. At a suitable position or altitude, and once the critical obstacles have been passed, the procedure will direct the crew to turn to the departure track.

[Lantirn]

Normally, we takeoff and maintain runway track to a suitable elevation and then turn to pick up the outbound course. If, however, a straight flight path departure is too restrictive for TOW, often we can find a curved, or turning takeoff departure, which gives a better RTOW and payload. Such turns can commence from very early in the departure, often at runway head, or at some specific position or altitude. There are various things which we take into account when designing such departures but, for the crew, the departure will be defined in the ops manual and the crew just follow what the book says for the runway. Flying a curved departure, though, is quite demanding as location and tracking is critical. As a consequence, speed and bank angle control is critical to keep the aircraft fairly close to the presumed tracking plan so that we don't get closer than intended to the obstacles of concern. At a suitable position or altitude, and once the critical obstacles have been passed, the procedure will direct the crew to turn to the departure track.

Now I understand. Its the wording and naming different for each one involved (performance engineer, regulator, manufacturer, and even differences between airlines) that made me not understand you from the beginning.

My company although not 100% correct uses the naming "Escape route" for engine out procedures after takeoff. Others use "EO route", "Emergency turn", "EO procedure" and the list goes on and on. So a curved departure would be an early turn of the "Escape route" to avoid something bigger straight ahead.

This is considered by the perf engineer on the construction of this maneuver, whereas "improved climb" ("increased V2" or "overspeed V2" as you mentioned it before) runs as an algorithm on the sofware to find a solution on that specific routing taking in account all the perf constraints.

By the way, as demanding is to fly an early turn for your EO route, its also demanding when the SID has a very early turn but your EO route goes straight or turns the other side. This applies more when you have an engine failure at low altitudes (around 300-600ft) which will make the crew to respond late while recognizing the problem etc

[Alex Whittingham]

Just to add a twist.. If on your first calculation (probably at the most limiting TOM so far calculated) you do not achieve the 35ft clearance on the net flight path then you have to reduce TOM. This has two effects, it improves the gradient but also it delivers a shorter TODR so the obstacle is further away from the end of the TODR, a point that used to be called reference zero. In olden days this was dealt with by making an arbitrary reduction in TOM, maybe 10,000 kg, replotting the net take-off flight path, observing more than the minimum clearance and then interpolating between the two weights to find an obstacle limited TOM. I don't know how the modern computer programs run the calculation but I have a suspicion that they don't move the end of the TODR as the weight reduces, but keep the obstacle distance fixed from the end of TODA. I would be interested to know if this is the way they calculate or not, if anyone knows?

[Lantirn]

I don't know how the modern computer programs run the calculation but I have a suspicion that they don't move the end of the TODR as the weight reduces, but keep the obstacle distance fixed from the end of TODA. I would be interested to know if this is the way they calculate or not, if anyone knows?

(Again, not a perf engineer here)

The obstacle distance in my EFB is calculated from the runway start (Brake Release Point)

Interestingly, when experimenting with different weights on dry and wet runways, I see on the expanded results that I can get 1st segment net heights lower than 35ft, on a dry runway, which confused me right away. To be specific, I can get 19ft on a dry runway.

If no one here have an obvious explanation for that maybe I will have to chat with our company perf engineer about that. john_tullamarine any clue about that?

[john_tullamarine]

"Escape route" as a term is fine and used commonly.

Overspeed (improved performance), equally, can be calculated manually, provided you have an AFM with the relevant data.

By the way, as demanding is to fly an early turn

Which is why the takeoff has to be briefed before you launch so that the crew know what they are intending to do. This doesn't protect you against every eventuality but reduces the likelihood of surprises.

I don't know how the modern computer programs run the calculation but I have a suspicion that they don't move the end of the TODR as the weight reduces

While one could use such a technique I see little reason for it. Whether running a manual or computer analysis, it is easy to incorporate the distance changes. After all, the aim is to extract the maximum weight for the conditions (I note that you, being Brit, use "mass" - apologies for our Antipodean slackness in using "weight").

but keep the obstacle distance fixed from the end of TODA

It is but a doddle to run that as the distance from brakes release.

maybe I will have to chat with our company perf engineer

That would be the preferred approach for a specific aircraft and analysis protocol. Doesn't surprise me, though.

[Alex Whittingham]

My suspicion comes from the Airbus "Getting to Grips..." publication. It notes (correctly) that an increase in V1/VR ratio moves the end of the TODR back and improves the obstacle limited TOM but for optimising V2/VS makes the statement "any V2/VS increase results in better climb gradients (1st and 2nd segment) and, therefore, in better climb limited MTOWs (1st segment, 2nd segment, obstacle)" Which I would agree with, except for the case of very close in obstacles where a greater V2/VS leads to a longer TODR and worse obstacle clearance until the improved gradient becomes dominant. Picky, I know, but I just wondered.

[Lantirn]

maybe I will have to chat with our company perf engineer

That would be the preferred approach for a specific aircraft and analysis protocol. Doesn't surprise me, though.

Asked and found out.

The height provided in the detailed analysis was additional to net height, its called reference zero. So that was 19ft above the net height of 35ft.

Another question I was thinking is, if the takeoff analysis stops at 1500ft, what happens if you lose an engine above 1500ft.

Do you have to switch to engine out, or you can continue to the SID? Technically you would continue to the SID, but if there is terrain I dont think its wise to do so. Better safe than sorry imho.

[Connie Wings]

Please, mates
Does anybody have this information or even any Boeing FCOM:
- How is the messages inhibition logic during takeoff phase ?

Thanks