Understanding takeoff performance

[italia458]

It seems incredibly difficult to self-teach takeoff performance! I have :ugh: many times so far. I'll try to clearly explain my troubles and hopefully somebody will be able to help me clear it up.

1) Does the climb gradient requirements under FAR 25 all refer to gross aircraft performance?

For example: under FAR 25.121(b)(1) it requires 2.4% climb gradient from gear retraction to 400' above the Departure End of the Runway (DER). Is that 2.4% gross or 2.4% net? According to this FAA document it appears that it is gross and the net is reduced by 0.8%, IAW FAR 25.115(b)(1), to a net gradient of 1.6%. http://i.imgur.com/E41Qwgd.png

2) My understanding is that FAR 25 has nothing to do with obstacle clearance and is only specifying a minimum level of performance that is required. Is that correct?

3) Under FAR 121.189 it details the net takeoff flight path clearing obstacles vertically by 35', etc. Is it correct then that, for obstacle clearance requirements, the net takeoff flight path for the 2nd segment would only be required to be 1.6%, and that gradient would be required to clear the obstacles by 35'?

I had always been under the impression that gross performance is the actual aircraft performance; net performance is the gross performance reduced by 0.8%; and that the performance requirements (gradients/accel distances) published in the FARs were referring to net performance - meaning that you are actually required to have a minimum of 2.4% net climb gradient in the 2nd segment.

4) My understanding is that a WAT (weight, altitude, temperature) chart ensures compliance with all FAR 25 performance requirements for takeoff. It is based on the most restrictive standard under FAR 25. Is that correct?

Thanks!

[Feno]

Hi,

2.4% is a gross climb gradient the manufacturer has to demonstrate the aeroplane complies with on the 2nd segment. [and provide the conditions under which said compliance is achieved]. This is a CS/FAR 25 certification performance requirement. The 0.8 % penalty is not related to that particular performance requirement.
CS/FAR 25.115 is not refering to the 2nd segment minimum climb gradient. It's refering to obstacle clearance. ie when there is a limiting obstacle, the 35 ft clearance has to be achieved after the penalty (0.8% for twins) is applied on whatever gross gradient you have in a given case.

2.4% is a minimum gross climb gradient when there is no limiting obstacle.
0.8% is a penalty that applies when there is an obstacle to clear

I think it may be worth to cross ckeck the source of the document you posted.
Cheers
Feno

[italia458]

Feno,

I think it may be worth to cross ckeck the source of the document you posted.

As you can see from the picture, the source seems to be the FAA. Below is a video where Coby Johnson, from the FAA, is giving a presentation using the slide that I referenced.

Airport Obstacle Analysis - YouTube (http://youtu.be/jj_IAjtE81A?t=13m33s)

[FlightPathOBN]

It is exactly FAA, and does a very good job of describing the criteria and many of the common misconceptions of obstacle clearance...

This is a fantastic explanation (and what I have been through as a certified RNP procedure designer) of the inconsistencies and misconceptions in the criteria, as well as the 1940's foundation.

This slide is one of the most relevant, (at 26:30) that I must keep trying to explain to pilots , procedure designers, and others, that departures and missed approach are designed all engine, and that engine out is NOT considered in the procedure design criteria, and as shown in the slide, that OE obstacle clearance is NOT covered in the procedures. (before you go crazy, note that IF the EO surface has been reviewed, through an exception, it may be shown on the plate)

http://operationsbasednavigation.com/wordpress/wp-content/uploads/2013/04/ScreenHunter_36-Apr.-08-15.04.jpg

note just as important, in using the DEP performance, that the criteria uses a linear, not a segmented path as shown in the cert...

http://operationsbasednavigation.com/wordpress/wp-content/uploads/2013/04/ScreenHunter_36-Apr.-08-15.17.jpg

[john_tullamarine]

(Unfortunately I am not able to access the video at the moment).

Your question is difficult only because you are picking bits and pieces out of the Standards.

Easiest summary might be

(a) expected climb performance should be not less than gross minima (and, in general, very marginally better than gross depending on the aircraft and engine condition)

(b) the AFM will provide a bunch of data for net degraded conditions

(c) obstacles must be below the net flight path as constructed iaw the AFM for the day. If the sums show this is not the case, then the take off parameters need to be varied to achieve the requirement .. either by reducing weight and/or increasing the speed schedule depending on what and where the problem is. This will result in climb gradients greater than the minima specified in the Standards.

For your specific questions

(1) second segment minimum gradient for a twin is 2.4% gross (1.6% net)

(2) FAR 25 has nothing to do with obstacle clearance

FAR 25 provides the tools to do the sums to miss the rocky bits.

Operational requirements (and commonsense) suggests that obstacles must be below the aircraft flight path. For the calculations in respect of obstacles, the relevant flight path is the net flight path.

Per 121.189(a) the AFM requirements are the starting point.

(3) FAR 121.189 ... the net takeoff flight path for the 2nd segment would only be required to be 1.6%, and that gradient would be required to clear the obstacles by 35'?

You're mixing things up a bit here.

The minimum net second segment flight path is 1.6% which provides a limiting weight.

Note that the limiting weight for the takeoff is the most limiting weight of the various limiting weights which have to be checked. That is, second segment limits may/may not be what limits the RTOW for the particular takeoff.

If the particular runway on the particular day is suited to the performance obtained at that weight .. fine .. otherwise you reduce the weight and/or increase the speed schedule to improve the gradient and, hence, obstacle clearance. That is the runway parameters provide additional input into the RTOW calculation.

Be careful not to consider climb limits in isolation to the other stuff which needs to be considered.

I had always been under the impression

(Caveat .. for twin engined aeroplanes). OK except that the minimum gradient requirement for second segment is 2.4% gross/1.6% net.

(4) WAT .. chart ensures compliance with all FAR 25 performance requirements for takeoff.

Not the case. The WAT data gives you the weight restrictions to meet the various minimum climb requirements

It is based on the most restrictive standard under FAR 25.

Not quite sure what you mean here ?

[FlightPathOBN]

The presentation is to illustrate the difference between what the cert states and surfaces, versus the actual areas and surfaces that are surveyed and taken into account in a procedure design. (ie the difference between the gross/net stepped perf vs the linear perf)

The FAA DEP procedure, and missed approach procedure designs use an all engine linear performance gradient for the obstacle clearance surface, which does not take into account the cert stepped performance EO.

As noted by the FAA, if you are DEP/MA EO, you may not be covered, and cannot rely on the published procedure unless it is specifically noted on the plate.

To follow JT, it is up to the operator, given the real time conditions, for EO DEP/MA, to determine the aircraft performance with procedure design criteria, not the cert criteria.

EDIT: To tell you the truth, one of the best ways to learn is DEP performance is to look at BCOP, the Boeing climb out performance program. This encompases all of the relevant issues such as aircraft performance, weight, altitude, and especially temperatures...

[HazelNuts39]

For example: under FAR 25.121(b)(1) it requires 2.4% climb gradient from gear retraction to 400' above the Departure End of the Runway Not exactly. 25.121(b) requires 2.4% gross climb gradient (for twins) "at the point where the landing gear is fully retracted" but without ground effect and at speed V2. The climb gradient reduces with altitude. Therefore at WAT-limit weight the average gross gradient between gear retracted and 400' will be somewhat less than 2.4%.

The take-off flight path requirements of 25.115 and the WAT-limit requirements of 25.121 are really quite separate requirements and should not be confused.

[italia458]

Hazelnuts,

The take-off flight path requirements of 25.115 and the WAT-limit requirements of 25.121 are really quite separate requirements and should not be confused.

So WAT limits are only related to 25.121 and no other FAR?

Therefore at WAT-limit weight the average gross gradient between gear retracted and 400' will be somewhat less than 2.4%.

So the gross gradient only needs to be 2.4% at the point the gear is retracted, and not a 2.4% gradient from gear retracted to 400'?

Flightpath,

I must keep trying to explain to pilots , procedure designers, and others, that departures and missed approach are designed all engine, and that engine out is NOT considered in the procedure design criteria, and as shown in the slide, that OE obstacle clearance is NOT covered in the procedures.

I think where that confusion starts, with pilots at least (myself included), is that we understand performance, in this regard, to be one item - climb gradient (climb rate and groundspeed). We will look at a departure procedure and see that it requires a climb gradient of x% which corresponds to a climb rate of say 500 fpm. If we look at our charts and see that the single engine rate of climb is >500 fpm for the conditions, why would we have to consider the departure procedure only being designed for all engine performance. In other words, if a Baron can fly the departure procedure with all engines operating and I can match the Baron's performance (climb gradient) while one engine inop, how is that any different? The procedure doesn't know if I'm performing with one engine inop or all engines - all that matters is climb performance (gradient).

Before you :ugh: I don't think that way anymore! I understand that all engine is a constant slope and OEI is a segmented procedure. OEI considers the level off to clean up, all engine doesn't. Vertical and horizontal clearance requirements are different between the two. Any other ones I'm missing?

John T,

You're mixing things up a bit here.

The minimum net second segment flight path is 1.6% which provides a limiting weight.

Note that the limiting weight for the takeoff is the most limiting weight of the various limiting weights which have to be checked. That is, second segment limits may/may not be what limits the RTOW for the particular takeoff.

So whatever the gross climb gradient the airplane will perform at that day, the net climb gradient will be reduced by 0.8%. For example, the gross gradient is 4.5%, therefore, the net gradient becomes 3.7% From a performance requirement - since the 4.5% gradient is greater than minimum required 2.4%, I can takeoff at a higher weight. From an obstacle requirement - I need to ensure the 3.7% clears the obstacles by 35'. I can decrease the net gradient to 1.6% in the second segment if that can clear the obstacles by 35', which means that I'm also at the minimum performance requirement for WAT limits (25.121). Say that a net gradient of 1.0% clears the obstacles by 35' in the second segment - I still can't use a weight higher than what would give a 2.4% gross climb gradient for the second segment.

Is that all correct?

To anyone,

Under FAR 25.115, it states that the net takeoff flight path is the actual takeoff flight path (determined in accordance with 25.111), reduced at each point by a gradient of climb equal to 0.8% for two engines.

FAR 25.111(c)(3)(i) states that at each point along the takeoff path, starting from 400', the available climb gradient must not be less than 1.2% for two engines.

Is the net climb gradient for determining obstacle clearance reduced to 0.4% then?

My understanding is that at 400', this available climb gradient is used to accelerate the airplane to Vfto.

Thanks for your patience! :)

[HazelNuts39]

So WAT limits are only related to 25.121 and no other FAR?The take-off WAT limit is the maximum weight that meets 25.121(a), (b) and (c), the landing WAT limit meets 25.119 and 25.121(d).

So the gross gradient only needs to be 2.4% at the point the gear is retracted, and not a 2.4% gradient from gear retracted to 400'?Correct.

Is the net climb gradient for determining obstacle clearance reduced to 0.4% then? That's not impossible - considering 25.121(c).

My understanding is that at 400', this available climb gradient is used to accelerate the airplane to Vfto.No, the acceleration (at 400' or greater height) corresponds to the available gradient reduced by the acceleration equivalent of 0.8 % gradient (twins) - see 25.115(c).

[john_tullamarine]

So whatever the gross climb gradient the airplane will perform at that day, the net climb gradient will be reduced by 0.8%.

That's the idea.

From a performance requirement - since the 4.5% gradient is greater than minimum required 2.4%, I can takeoff at a higher weight.

Your use of terminology probably is part of your problem. "Performance" relates to the whole gamut of how the aeroplane does things in respect of accelerations, decelerations, climbs, descents, turns, and so forth.

Talking about gross and net climb gradients is only ONE part of the performance story.

Better to think in terms of .. than minimum required 2.4% under WAT -limiting conditions in the second segment.

So, if you are are at a weight, on the day, which might produce 4.5% gross in the second segment configuration, then you MAY be able to increase weight. Two extreme situations to consider .. and a range in between -

(a) if there are NO obstacles (eg really flat country or overwater, ignoring transient objects), the runway weights were not limiting, and you didn't have other eg ATC restrictions to consider, then you would be able to increase weight until you reached the relevant WAT limit.

(b) if there are obstacles and you are already limited by one or more of those obstacles, then the WAT limit ceases to be relevant and you can't increase weight.

(c) in the more general case, you will be able to increase weight UNTIL one of the various calculation cases becomes limiting. At that point you are stuck with whatever weight you have. Keep in mind, also, that the calculation which is limiting may CHANGE throughout the takeoff.

From an obstacle requirement - I need to ensure the 3.7% clears the obstacles by 35'.

.. for the period you are in the second segment.

I can decrease the net gradient to 1.6% in the second segment if that can clear the obstacles by 35',

.. for the period in the second segment.

which means that I'm also at the minimum performance requirement for WAT limits (25.121)

yes for the first bit. However, as others have observed, treat Parts 25 and 121 as two parts of a whole. WAT limits are part of the aircraft certification and built into the AFM. Part 25 addresses building an aeroplane which, ultimately, gets onto the aerobridge (using a bunch of other rules) for the pilot to use. Part 121 largely tells the pilot (operator) what he may/may not do from that point in flying from A to B.

Say that a net gradient of 1.0% clears the obstacles by 35' in the second segment - I still can't use a weight higher than what would give a 2.4% gross climb gradient for the second segment.

Right. You can calculate a weight to give 1.0% net but you can't use it in practice as you are restricted (both by Part 25 AND Part 121) to whatever the WAT limit might be on the day. Just means that the pilot doesn't sweat the takeoff quite as much ...

I think it's easier to reflect in terms of

(a) gross climb approximates what the pilot should see on a reasonable day (lots of things can reduce the actual climb, eg turbulence, inversions, incorrect piloting technique, etc .. which is why we have)

(b) net climb as a calculated, conservative construct to give us a reasonable chance of missing the rocky bits on the day when it counts most. On very, very rare occasions, you might be a statistical outlier and, despite all your very best efforts .. still end up making the CFIT pages. No guarantees in flying .. only statistics.

[Natstrackalpha]

Will the climb/gradient capability of any particular aircraft be printed in the docs of the aircraft, which docs please?

Or, is there a universal doc like the one one figures out V1 . .and . . .everythingelse for perf A? . . based on WAT, and the rest of it, runway perf . . . etc., etc., etc.

[italia458]

Hazelnuts and John T,

Thanks for you help! I think I'm understanding it now.

Keep in mind, also, that the calculation which is limiting may CHANGE throughout the takeoff.

If I'm already on the takeoff roll at the limiting weight for performance and obstacles, how would the factor that's limiting change while on the takeoff? Are you considering an engine failure past V1?

I really wish the regulations for Air Taxi and Commuter ops in Canada were as restrictive as an airline operation with Transport category airplanes! In Canada, you can takeoff in a 16,000 lb twin-turboprop airplane (specifically the Fairchild Metro III [SA227-AC]) with 2 crew and 9 passengers (operating as an Air Taxi), without satisfying ASDR, TODR, TORR, and without satisfying obstacle clearance in the event of an engine failure. Still need to satisfy the WAT limits, but the second segment is reduced to 2.0% instead of 2.4% for commuter category airplanes as per CAR 523. You'd also have to ensure you'd be able to maintain the MOCA with any one engine failed, but only if IFR or night VFR. Air Taxi ops in Canada has zero regulations/standards under "Aircraft Performance Operating Limitations" - Commuter ops in Canada has regs/standards similar to Airline ops under "Aircraft Performance Operating Limitations". If anyone is familiar with the Canadian regs, correct me if I'm wrong.

It seems good airmanship would dictate that you plan an escape route for departure if you determine you can't clear obstacles with OEI but it seems it'd be legal to takeoff knowing that you would crash into a mountain if you had an engine failure. One Canadian regulation states: "No person shall operate an aircraft in such a reckless or negligent manner as to endanger or be likely to endanger the life or property of any person." I don't think that applies here though. I wonder if that passengers would want to know this...

Any thoughts on this?

[italia458]

Natstrackalpha,

For airplanes with certified engine-out performance, that information will be contained in the "Performance" section of the AFM.

[Natstrackalpha]

Grazie. Italia458

[john_tullamarine]

how would the factor that's limiting change while on the takeoff?

Reflecting, I probably should have been a little more precise with my words viz., at different locations in the RTOW data set, the limiting case may/will vary.

For a particular takeoff with specific parameters, there will be a particular limiting consideration. Apologies for the needless confusion ...

In the basic method of analysis (ie what we did in the olden days before everyone bought themselves a pocket gadget which rivalled mainframes of old) ..

(a) each particular limit was calculated and the limiting weights plotted on a bit of graph paper .. producing some sort of line(s) defining the weight limit variation against temperature (as the usual presentation).

(b) once all this stuff was done .. and it could take quite a while for a complex runway and AFM ... the graph paper was covered with a bunch of lines.

(c) one then rubbed out all the non-limiting line segments ie ending up with the lowest limiting weight at each temperature point.

(d) it was not unusual to find the limiting case changing several times as one went from cold to hot .... all depended on how the original lines fell to paper.

Unfortunately, the younger set in the Ops Eng fraternity run electronic wizardry these days and don't get to play with the olden days techniques .. which gave a wonderful sense of understanding feel for how an aircraft did this and that.

I really wish the regulations for Air Taxi and Commuter ops in Canada were as restrictive as an airline operation with Transport category airplanes!

Ideally, the rules reflect the reality of what can be achieved considering the feasibility of the communities served being able to afford the service. In a similar manner, Australia (when it had responsibility for PNG) introduced very much lowered standards (compared to Oz) reflecting the very different situation existing in PNG.

Horses for courses is the expression we'd tend to use out in the Colonies.

However, it doesn't preclude a risk aware corporate organisation from adopting higher than minimum standards .. however unlikely that may be in the real world.

It seems good airmanship would dictate that you plan an escape route for departure if you determine you can't clear obstacles with OEI but it seems it'd be legal to takeoff knowing that you would crash into a mountain if you had an engine failure.

I refer you back to my previous statement. No real advantage being legal and dead simultaneously. Hence the reference to sensible corporate risk management.

In a long ago previous life I was responsible for Ops Eng for a jet operator .. the Chief Pilot and I, in the most dreadful collusion, sometimes introduced regulatory conservatism .. the better to allow both of us to sleep well at night. What the bean counters didn't know didn't fuss them. Obviously we didn't do this sort of thing capriciously but we did give due regard to real world commonsense.

"No person shall operate an aircraft in such a reckless or negligent manner as to endanger or be likely to endanger the life or property of any person."

Aye, therein lies the rub .. as in most sets of Aviation Regulatory writings.

It probably would take a comment from Flying Lawyer, or one of his PPRuNe legal colleagues, to clarify just what the words mean precisely at law .. however, I have always found a degree of healthy conservatism to be a reasonable and protective defence against predatory legal activity after the event ..

[FlightPathOBN]

Here are a few dots to connect.

This data is for the missed approach performance profile, 737-700, and since as this is for a procedure design, uses all of the worst case variables, EO, max landing wt, 5 sec engines, icing/bleeds, etc...begin missed at 140kts.


700 perf (NA below)
Initial climb (net): 2.35%
Initial climb length: 5.7nm
Level off length: 5.2nm
Second climb (net): 2.77%
Second climb Length: 26.5nm
Third climb (net): 1.85%

700 perf (NA above)
Initial climb (net): 1.20%
Initial Length: 11.24nm
Level off length: 17.6nm
Second Climb (net): 2.10%
Length: 35.1nm
Third climb (net): 1.91%

[Natstrackalpha]

Wow!

I wonder if they still teach perf A like they used to - it was a real headbanger.
J.T You are on my list of demi-gods.

Great thread by the way. Beautifully started by 458

[john_tullamarine]

.. now, how to get my wife to view me in like manner ?

I suspect that her views, probably, are somewhat more pragmatic ..