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Why do we have take-off segments?

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Why do we have take-off segments?

Old 6th Mar 2013, 17:01
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Why do we have take-off segments?

I was asked this recently and I have no idea. Google didn't return much either. My only guess was it's a convenient way to describe it and talk about it?

Secondly, the first segment. My manuals say it starts from the end of the TODR to gear up but I was always taught it's lift off (so basically the end of TORR) to gear up. Which is correct? Is there a difference?

Last edited by pudoc; 6th Mar 2013 at 17:04.
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Old 6th Mar 2013, 17:29
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Different rules (required climb gradients etc.) apply to different parts of the take-off flight path: segments. If you had ever calculated obstacle clearance TOWs by hand, you'd never forget!

Airbus says the first segment is from the 35' height point up to gear retraction.

Last edited by toffeez; 6th Mar 2013 at 17:43.
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Old 6th Mar 2013, 17:37
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There is a difference, albeit of academic nature... TOD is up to screen height, where TOR is to the point where the wheels leave the ground.

The segments are a logical way of describing the various phases that most aircraft go through during a departure. Gear up and clean up are such events that define the various phases and each event will affect performance in its own way. Therefore, why not let these events define the segments so that also the performance requirements may follow? I think its logical at least.
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Old 6th Mar 2013, 19:58
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We have them so we don't run into Terra-Firma.

Airports tend to be built on relatively level ground, so we devise a system whereby we take advantage of that to get up and get the aircraft clean and at Vy- then we out-climb the "Hill" off the end of the runway.
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Old 6th Mar 2013, 20:12
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A lot of confusion arises from the fact that the take-off performance requirements of CS-25 do not define any 'segments' as such. Breaking the sometimes confusing requirements down into four segments is just a way of arranging things in a convenient order, so one can discuss them more easily.

The first segment, for example, is defined in CS 25.121(a) as 'the critical take-off configuration existing along the flight path (between the points at which the aeroplane reaches VLOF and at which the landing gear is fully retracted)'. As you can see, there is indeed a reference to VLOF, but since the take-off flight path begins at 35 ft 'above the take-off surface at the end of the take-off distance' [CS 25.115(a)] the first segment begins at the screen height.

TOR is to the point where the wheels leave the ground
I used to think it was halfway between this point and the point where the screen height is reached, respectively 1.15 times this distance for the AEO case.
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Old 6th Mar 2013, 21:35
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Some thoughts, if I may ...

Why do we have take-off segments?

Repeatability. By prescribing some sort of how to do it from A to B in a very simple, mechanical way, we can define technique, measure progress and deviation from the plan, etc.

Otherwise it would be a bit of a shambles and not the sort of thing we look for in heavy iron operations.

first segment

the takeoff involves getting off the ground and then selecting gear up (iaw the AFM - might be, for instance, two seconds after liftoff - whatever). Depending on the performance on the day and gear sequencing, the gear will be tucked up and away after a period of seconds at which stage the aircraft will be at whatever height ....

Should this process finish prior to the end of the TODR, there is no first segment and we start with the second segment from the end of the TODR (ie screen) - typically hot, high, heavy ie poorer performance.

Should this process finish after the end of the TODR, the distance between end of TODR and gear eventually up is the first segment - typically cold, low, light ie better performance

Some aircraft have a retract cycle sufficiently long winded that it doesn't make any difference and there is always a first segment ....

My manuals say it starts from the end of the TODR to gear up

correct

but I was always taught it's lift off (so basically the end of TORR) to gear up.

Suggest you change instructors

Is there a difference?

Certainly is - the flare distance to screen.

TOR is to the point where the wheels leave the ground

Common misconception.

TORA is declared by the airport folks and is just a number relating to runway construction details

Depending on the rules in play, TORR is either

(a) distance from start of takeoff to one-half the distance between lift off and screen, or

(b) distance from start of takeoff to one-third the distance between lift off and screen - probably no longer in vogue anyway, I'd guess

ie the aim is to give you a high probability of getting off the ground prior to the end of the higher bearing strength bits as defined.

Breaking the sometimes confusing requirements down into four segments

Minor point but, occasionally, you will find aircraft which use 5 segments
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Old 6th Mar 2013, 23:10
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Just musing. Is it correct to say that 400 ft is the minimum height for start of third segment because otherwise the lower you get the more risk of flying back into the deck during flap retract?

Or another view is the sooner you get the aircraft cleaned up, the better it will climb single engine (two engine types) and generally better obstacle clearance. This would lead inevitably to some pilots chancing their arm and cleaning up at less than 100ft with all its attendant risks (especially night) in order to realise higher weights.

On the other hand, it is now common for manufacturers to recommend 400 ft as the minimum "safe" height to be attained before actioning serious check list items such as engine severe damage and/or fire warning. Quite a coincidence that the minimum certification height for start of third segment is 400 ft and which miraculously also becomes the minimum "safe" altitude to start emergency checklist items. I have often wondered about that rather convenient relationship. Maybe it is just to simplify things?

Last edited by Centaurus; 6th Mar 2013 at 23:12.
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Old 6th Mar 2013, 23:16
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Is it correct to say that 400 ft is the minimum height for start of third segment because ..

Can't recall ever having read the definitive history. However, it will be found in some dusty ICAO tome of the 50s which led to its embodiment in the Design Standards.

the sooner you get the aircraft cleaned up, the better it will climb single engine (two engine types) and generally better obstacle clearance.

.. but don't hit the ground whilst doing so ...

On the other hand, it is now common for manufacturers to recommend 400 ft as the minimum "safe" height to be attained before actioning serious check list items

I've always presumed that this development was solely from considerations of standardisation .. ie concentrate on flying intially to get away from the ground and, as some appropriate level, the presumption is that there will be a more sensible time to worry about the side issues ...
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Old 7th Mar 2013, 08:57
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Thumbs up 3 questions about Takeoff Segments

Hiya' all,

I hope this is the righ thread to post this:

I have 3 questions I came up with about the Takeoff Segments:

1. I was asked by a friend a legal document that summerises all 4 takeoff segments: where each starts and ends, speeds to be flown in each one, power settings in each one, required climb gradients, etc..
Where can I find such a thing?

And with this regards (maybe), I see a lot that CS 25 is being mentioned. What is CS 25?

2. As far as I know the $th segment (Final Takeoff Segment) ends either at 1,500' AAL (Above Aerodrome Level) or when CLEAR OF OBSTACLES.
(I assume which ever is higher).
How is CLEAR OF ABSTACLES defined for this purpose? Bt what obstacle clearance?

3. Are all 4 segments relevant both in Normal operations AND Single Engine Operations or just when loosing an engine at V1?
Are there any differences between the 2 situations?


Thanks a bunch!

Last edited by yonygg; 8th Mar 2013 at 05:52.
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Old 7th Mar 2013, 09:53
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yonygg

Here is some light reading for when you're bored:

Getting to Grips With Aircraft Performance

Start with page 63.

Last edited by toffeez; 7th Mar 2013 at 09:56.
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Old 7th Mar 2013, 13:54
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John
Can't recall ever having read the definitive history. However, it will be found in some dusty ICAO tome of the 50s which led to its embodiment in the Design Standards.
That was a challenge, so I looked out my treasured copy of the 1953 ICAO Final Report of the Standing Committee on Performance. It's there, but you aren't going to believe this!

They were trying to establish statistical margins for a range of aircraft to define how much height should be allowed so that at any point on the Net Take-off Flight Path the probability of being below the NFP would be equal to the design incident probability. To do this they calculated the gross/net margins at a range of heights for a "few typical aeroplanes" and found that the resulting NFP was curved. To simplify things without introducing too much conservatism they decided to approximate the curved path by two straight lines and guess what? the convenient break point was 400 ft.

This matched some unspecified administrative feasibility and got enshrined into the standards.

Subsequently, since some of the aircraft they studied couldn't reach 1000 ft (OEI) before they ran out of time at TOP, the rules were amended to allow a change of configuration after 400 ft.

Incidentally, that same report says that the datum height at the end of the Take-off Distance Required is clearly a point on the NFP, and that since the height of the ground beyond this point is uncertain no credit for ground effect on performance can be given.

Amazing how myths develop isn't it
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Old 7th Mar 2013, 15:25
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Very interesting indeed,thanks for digging.

Last edited by de facto; 7th Mar 2013 at 15:25.
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Old 7th Mar 2013, 16:50
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CliveL you are now my favourite pprunner
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Old 7th Mar 2013, 21:34
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They were trying to establish statistical margins for a range of aircraft

That was the gist of the story given to me by John Walshe (ops eng boss - absolutely lovely bloke) at Ansett in the late 60s .. pity he didn't give me the document to read in toto.

approximate the curved path by two straight lines and guess what? the convenient break point was 400 ft.

ah, now that I didn't know .. thank you
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Old 7th Mar 2013, 21:49
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Depending on aircraft type clean up altitude was predicated on the 5 minute takeoff power to selecting climb power after flaps up and reaching min speed for clean configuration. The B727 was lower than the 757.
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Old 7th Mar 2013, 23:38
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.. and you see other odd ball things. eg those RR Dart powered gracious ladies of the air generally are limited to a max third segment of 600ft due to a time limit on the feather pump.
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Old 7th Mar 2013, 23:38
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Originally Posted by CliveL
To simplify things without introducing too much conservatism they decided to approximate the curved path by two straight lines and guess what? the convenient break point was 400 ft.
To do their statistical analysis, they assumed an average takeoff technique.
The Average Take-off Technique assumed is not different in principle from the technique assumed in the previous ICAO code. (...) The climb is assumed to be made in straight flight up to a height of 400 ft at the Take-off Safety Speed; then turns, accelerations and changes of configuration are allowed; however, the possibility of turns being made before the height of 400 feet is reached is envisaged ...
EDIT:
In the SCOP Proposed Standards (Part IV of the report) the 'nettage' deduction from the gross path is 0.65% below 400 ft and 0.95% above 400 ft (for twins at 2*10^-6 IP). The 'administrative feasibility' is that the height chosen for the nettage change corresponds to the minimum acceleration height in the assumed Average Take-off Technique. (If one overlooks that 400 ft net has become 400 ft gross in Part IV).

The concept of variable nettage was not adopted in current regulation. In 1957 the US CAA/CAB published Special Civil Air Regulation No. SR-422 which specified that the gross FP must clear obstacles with a vertical margin of 35 ft plus 1% of the distance travelled from end of TODR.

Last edited by HazelNuts39; 8th Mar 2013 at 12:35.
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Old 8th Mar 2013, 14:06
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Back to OP

Originally Posted by pudoc
Secondly, the first segment. My manuals say it starts from the end of the TODR to gear up but I was always taught it's lift off (so basically the end of TORR) to gear up. Which is correct? Is there a difference?
It depends whether you are talking of the take-off path (25.111) or the take-off flight path (25.115). The take-off flight path starts at 35 ft at theend of the TODR (25.113(a) and (b)). The first segment of the take-off flight path is from that point until the point where the landing gear is fully retracted.

The take-off path starts at the beginning of the takeoff and may be determined either from continuous demonstrated take-offs or by synthesis from segments. If the segmental method is used, the first airborne segment extends from lift-off to the point where the landing gear is fully retracted.

P.S.
If the segmental method is used, the airborne part of the take-off is based on free-air data without ground effect. If the continuous demonstrated take-off is used, it is based on the demonstrated performance until the airplane is out of ground effect.

Last edited by HazelNuts39; 8th Mar 2013 at 14:26. Reason: P.S. added
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Old 8th Mar 2013, 18:27
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The reason for the segments is the requirements.

If you require something, you have to be specific.

In an engine out situation you need to climb, accelerate and clean, and authorities require minimum climb gradients for each part o the process:

1 when you still have the gear down,
2 when you have it up already,
3 when you accelerate in level flight and clean up (the acceleration rate required is the equivalent to a climb gradient requierement), and
4 when you are clean and climbing with MCT

For each stage they make a different requirement, hence the 4 segments

This is for climbing requirements, not for obstacle. For obstacle you have to deal with the net flight path, the DER, and all that stuff.
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Old 8th Mar 2013, 21:17
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Microburst 2002

Thanks for the explenation Microburst.
Can you also expand your explanation about the obstacles requirements?
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