Why do we have take-off segments?

[pudoc]

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?

[toffeez]

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.

[low n' slow]

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.

[Wizofoz]

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.

[hvogt]

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.

[john_tullamarine]

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

[Centaurus]

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?

[john_tullamarine]

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 ...

[yonygg]

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!

[toffeez]

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

Getting to Grips With Aircraft Performance (http://www.smartcockpit.com/aircraft-ressources/Getting_to_Grips_With_Aircraft_Performance.html)

Start with page 63.

[CliveL]

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 ;)

[de facto]

Very interesting indeed,thanks for digging.:ok:

[FE Hoppy]

CliveL you are now my favourite pprunner :ok::D

[john_tullamarine]

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

[bubbers44]

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.

[john_tullamarine]

.. 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.

[HazelNuts39]

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.

[HazelNuts39]

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.

[Microburst2002]

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.

[yonygg]

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

[vilas]

Minimum acc ht of 400ft is a regulatory requirement both JAA/FAA and therefore minimum operational acceleration height is 400 ft or higher

[jmn]

Hi all,

2.4% minimum gross gradient; can it be increased?

[john_tullamarine]

2.4% minimum gross gradient; can it be increased?

Simply by reducing weight which often is required for obstacle clearance.

[de facto]

Simply by reducing weight which often is required for obstacle clearance

Before off loading bags,use of improved climb may be used..

[Feno]

2.4% minimum gross gradient; can it be increased?


If I may, 2.4% is the minimum net gradient requirement on the 2nd segment for twin engines.

To answer the question:

Increasing V2 and/or reducing flaps setting can be considered if TODA is not limiting.
Otherwise, like already said, mass reduction is the other option.

[HazelNuts39]

2.4% is the minimum net gradient requirement on the 2nd segment for twin engines.Wrong. It is a gross gradient. It is obtained at the start of the 2nd segment. The gradient of the flight path reduces with height due to the thrust reducing with altitude. Therefore WAT-limit gradient is not equal to the average gross flight path gradient in the second segment.

A lot of confusion arises from the fact that the take-off performance requirements of CS-25 do not define any 'segments' as such.I think the confusion arises from failure to appreciate the differences between the take-off climb requirements of 25.121 and the gradient of the flight path defined in 25.115.

25.121(a), (b) and (c) contains three minimum steady climb gradient requirements out of ground effect in still air which together define the maximum take-off weight for the altitude and temperature, or WAT-limit. That is an airworthiness limitation that exists independent of the presence of obstacles. It replaces a provision in the predecessor regulation (CAR 4b) that required a minimum rate of climb as a function of the stall speed prior to the age of turbine engines.

In the take-off flight path the gradient is never steady. The ground effect changes with height above the surface. The drag changes when the configuration or speed changes. The thrust changes with ambient pressure and temperature as the airplane climbs, and with changes of airspeed. The take-off flight path is also subject to wind.

25.121(a) takes speed, configuration, and thrust at different points of the 'first' segment:
- speed is at liftoff,
- configuration is at the point of maximum drag in the gear retraction cycle - typically wheel well doors open and the gear half-way retracted,
- thrust is at the point where L/G retraction is begun - typically three seconds after liftoff.

[Feno]

Thx for clearing that up.
The gross ("actual" as per CS 25.115 terminology) and net notions apply as part of the clearing obstacles process by 35 ft, which is a separate issue from the gross 2.4% requirements for which the 0.8 % penalty would not make sens then.

[Roger Greendeck]

Part of the confusion comes from what the rules are written for.

The segments are, initially, for certifying the aircraft. Hence they are spelled out in detail in the FARs. Naturally if an aircraft is certified against certain requirements that manufacturer will have generated the flight test data to prove their case. Thus they can publish this data for the user and if we can fly the same profile and have certainty of the aircraft's performance.

If you are flying an aircraft with marginal performance then you will most likely have to fly the same segments to be able to ensure terrain clearance. If, however, you are blessed with additional performance then you could safely, from a terrain clearance perspective, fly a different profile such as accelerating early or retracting flaps early.

Just to add to the confusion we now take into account crew workload, automation, and noise abatement when designing procedures. So to the casual reader of a company ops manual it may not be clear why events happen at a certain point. Myths and legends then appear that 'you can't' do certain things.

For example, in Australia the requirement is to ensure 35' terrain clearance on departure or 50' in a turning departure. There is no regulatory requirement to fly a segmented climb but most operators have a segmented climb procedure. But this does not stop you flying a profile of your choosing if you maintain the required terrain clearance.

[john_tullamarine]

But this does not stop you flying a profile of your choosing if you maintain the required terrain clearance.

quite so .. but how do you establish (ie where are the numbers and hard data for your benefit at the Inquiry) that all is OK before you launch ? The advantage of flying it per the AFM is that the OEM has done all that stuff.

Different matter AEO to keep yourself above the OEI net profile ..

[Feno]

Part of the confusion comes from what the rules are written for.
Agreed

It's quite often a certification vs exploitation confusion.
Another example being the GA approach climb 2.1 % climb gradient requirement which is a certification one as opposed to the 2.5 % climb gradient imposed by OPS 1.510 for approaches with Decision Heights lower than 200 ft.
On a side note, about that particular last point, Jeppesen doc seems to use the 2.5% climb gradient requirement as a standard figure for any type of approach, whereas even Cat 1 approaches are not subjected to it (their DH's never being lower than 200 ft). Hence any Company using exclusively jeppesen figures in this particular case potentially penalize themselves unnecessarily.

While I'm at it, here's another one:
About the previously discussed points of take off gross/net/penalty climb gradient in the scope of obstacle clearance, I've failed to find any penalty requirement when it comes to GA obstacle clearance requirements. Just would like to have a clear confirmation that they actually don't exist.

[LeadSled]

Minimum acc ht of 400ft is a regulatory requirement both JAA/FAA and therefore minimum operational acceleration height is 400 ft or higher

Folks,
I would suggest the above is late in the chronology of developments.

Re. the derivation of the original 400', a most interesting piece of information, the first time I saw the results as a certification standard was SFAR 422B (B707 etc). SFAR 422B preceded the establishment of FAR 25, and a JAR was something with jam in it.

Given the T/O power time limits for such as the JT-4 (original B707-300 series), 400' fitted the time limits, with max oil temperature as well as max EGT being an issue.

The general increase to a minimum of 800' for power reduction, acceleration and configuration change was influenced, as I recall, by the loss of a South African Airways B707 departing Windhoek, Namibia.

[Roger Greendeck]

John,

You are quite right that unless you have the data you can't plan your profile in advance. So in practice we end up using profiles very similar to those for certification. My point was a semantic one about what is legal or not rather than the practical application. If you have the data you can legally use a different profile.

[vilas]

4.1.3.1. MinimumAcceleration Height

JAR 25.111 Subpart BFAR 25.111 Subpart B

“JAR/FAR 25.111

(c)(2) The aeroplanemust reach V2 before it is 35 ft above the takeoffsurface and must continue at a speed not less than V2 until it is 400 ft above the takeoff surface”


4.1.3.2. MaximumAcceleration Height

The Maximum Takeoff Thrust(TOGA) is certified for use for a maximum of 10
minutes, in case of anengine failure at takeoff, and for a maximum of 5 minutes with
all engines operating.The Maximum ContinuousThrust (MCT), which is not time-limited, can only be selected once the enrouteconfiguration is achieved (i.e. when the aircraft is in clean configuration at green dotspeed). As a result, the enroute configuration (end of the third segment) must be achieved within a maximum of 10 minutes after takeoff, thus enabling the determination of a maximum acceleration height
Reproduced from Airbus document. GETTING TO GRIPS WITH PERFORMANCE

[FE Hoppy]

5 or 10 minutes is engine specific regardless of the generic airbus publication.


On the little ejets it's just a matter of buying the upgrade from embraer.

[HazelNuts39]

Feno,

Airplanes operating under EU-OPS will have been certified under EU regulations. The EU certification requirements for Decision heights less than 200 ft are contained in CS-AWO (All Weather Operations), which essentially requires the AFM to provide a WAT-limit chart for 2.5 % gradient at the speed used for go-around.

Vilas,

The quote from FAR/JAR 25.111(c)(2) is incomplete. It omits the essential bit which reads:
"... and must continue at a speed as close as practical to, but not less than V2 ...".

[italia458]

A little over a year late but I thought I'd post this document here since it explains many of the details that people in this thread are inquiring about.

It's about the history of the regulations and explains some of the reasoning behind the current regulations.

http://flightsafety.org/fsd/fsd_feb00.pdf

[soundlover]

Wow! This thread has became one of the most interesting and constructive threads I have ever seen on a forum.
Thank you guys

[mikedreamer787]

This should clear up about first and second segments -

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