Why the acceleration height is minimum 400 ft, not lower?

[Buncha24]

I just wonder. When one engine fails after V1 and go, we have to reach 400 feet minnimun before we can accelerate. The problem is, why it can't be lower than 400 feet? :confused:

[NSEU]

The experts probably deemed it safer to gain height above the runway first, giving you some kind of safety margin if there was windshear.

You should have already attained enough speed to handle the loss of an engine in the first hundred feet or so, so the priority switches to maintaining terrain clearance.

[Empty Cruise]

You'll find that many companies don't accelerate @ 400, even if no obstacles are present. Most use 400 ft. as the limit to initiate recall items and only accelerate when you've had a reasonable time to execute these, say, at 1000 ft AAL or higher.

[john_tullamarine]

Keep in mind that the 400 feet third segment is gross while terrain calculations are based on net.

If you play with the geometry, 400 feet gross, for a simple case, gives you something in the vicinity of 280 feet net for a twin (according to my back of a fag packet calculation). If you start pushing down the 400 feet gross, you end up with a need for very little in the way of terrain to make the thing work.

At the end of the day, it's just a line in the sand sort of number.

[batman123]

no ECAM action below 400ft .

[captjns]

Some carriers would to start the acceleration at 600’. Presumably, the aircraft would be above 700’ by the time the nose has been lowered, which is above the “DON’T SINK’ alert feature.

Mode 3 of the GPWS provides an alert if a decent is made during initial climb or go around to 700’ AGL. The aural alert is a voice message "DON'T SINK", and is repeated until the condition is corrected. It is effective between 50 and 700 feet radio altitude and generates the alert when accumulated barometric loss equals approximately 10% of the existing radio altitude. Mode 3 does not arm during decent until below 200 ft radio altitude.

[411A]

It's all very simple.

Long before there was a JAA/EASA/FAR....there was CAR4B, which established 400 agl as the end of the second segment.

Carryover from the old days...and a good one at that.


However, with piston transport aircraft, 50 feet was the screen height, yet 35 feet was established as the screen height for jet transport aircraft, by CAR4B

Why, you ask?
That question is a bit more complicated, however...the basic facts are that the performance of the early jet transports was many times so poor, and the runways available so short....that it was impossible for the 50 foot screen height to be achieved, at the max weights anticipated.

I flew one of these early poor performers for quite awhile, the B707-320, and at its max weight (317,000 pounds), up close and personal with the far end of the runway whilst becoming airbourne, was a daily occurance...with all 4 turning, let alone with one engine unserviceable.

[Flyman35]

I agree with captjns (http://www.pprune.org/members/129678-captjns)

will we don't use here 400 feet as SE ACC ALT however it is the minimum Alt to start actions,where it is guarantee a stabilized and safe trajectory and it is an appropriate compromise between stabilizing the aircraft and delaying actions. (reference)A318/A319/A320 FLIGHT CREW TRAINING MANUAL _OPERATIONAL PHILOSOPHY 01.040 PAGE 5 OF 10.

[kwachon]

AIM Chapter 5 Section 2

Obstacle clearance is based upon an aircraft climbing at least 200 ft/nm crossing the end of the runway by at least 35ft agl, then climbing to 400ft above the airport elevation before turning unless otherwise specified in the departure procedure.

FAA Part 25

At V2 with 1 engine out, you are guaranteed 2.4% gross, 1.6% net climb gradient. 400ft is the start of the third segment of the climb profile certification.

Cheers

KW

[Mansfield]

Specifically, FAR 25.111 states that:

The airplane must reach V 2before it is 35 feet above the takeoff surface and must continue at a speed as close as practical to, but not less than V 2, until it is 400 feet above the takeoff surface;

and

The airplane configuration may not be changed, except for gear retraction and automatic propeller feathering, and no change in power or thrust that requires action by the pilot may be made until the airplane is 400 feet above the takeoff surface;

The 400 foot idea seems to have been introduced in SR 422, which was the 1957 change to certification in anticipation of turbine powered transports. I don't see it referred to in CAR 4b as it was in 1953.

This gives me the chance to promulgate my pet theory about the change from a 50 foot screen height to a 35 foot screen height. I have never been able to find a true reason for this, but consider that, a) Boeing had bet more than the net worth of the company on the 707 project, b) the 707 in its original form couldn't make a 50 foot screen when operating from many of the existing runways in the U.S. (that had been designed for piston transports), and c) Boeing was the prime contractor for some very significant defense projects at the time (B-52, Bomarc, etc.)

At the risk of opening the Pandora's box of government subsidization and the like, I have often wondered whether it simply came down to making sure the 707 worked so that Boeing stayed solvent in the middle of the Cold War. I have absolutely no grounds for this theory, but it used to be a fun story to use when teaching performance to turboprop (50 foot screen height) pilots back in the eighties.

[SNS3Guppy]

Our minimum level-off with an engine-out is 800', but may potentially be higher with an airport analysis. We don't normally touch flaps until 3,000'.

In some of my light airplane flying, the airplane is accelerated in ground effect while flaps are retracted, and then a turn on course made often before the end of the runway.

[Mansfield]

Another consideration in this discussion is that the third segment is defined as being flown with takeoff power still set. The time limit on takeoff power becomes a limiting factor if obstacle clearance requires a higher acceleration height. If, for example, the manufacturer specifies a five minute limit on takeoff power, then the second segment time to acceleration height is limited based on the time required for the third segment acceleration and cleanup.

[powdermonkey]

Just a quick question

1st segment = 35ft screen to gear up
2nd segment = gear up to anything from 400ft-1000ft? is that correct
3rd segment = end of 2nd segment to flap retraction altitude.

Bit confused on the 2nd segment....

[DA50driver]

The 400' level off was to allow the old water injected jets get up to speed before the water ran out. Without water the airplane would not be able to accelerate, and would come back down to earth again. (I always wondered about this myself, finally got the answer from someone a lot more experienced than me).

[411A]

...and would come back down to earth again.

A bit of a stretch, however, they would certainly descend during the third segment, which is not allowed.
The water flow in the Boeing airplane lasted for approximately 2.5 minutes, and it was allowed to do so until the water tanks were empty.
Been there, done that...:}

[Mansfield]

PM,

1st segment - 35 feet screen to gear up (turbojet only; 50 feet otherwise)

2nd segment - gear up to acceleration height (min 400 feet), flown at takeoff power and V2 to V2+15

3rd segment - acceleration height to clean configuration, flown at takeoff power and minimum climb rate

4th segment - clean configuration to 1500 feet, flown at maximum continuous power and clean min maneuvering speed

At least, that's as far as I understand it based on the Boeing AFM...but if I've learned anything in this business, it is that there is always a exception.

[powdermonkey]

Ah cheers Mansfield
I was looking up old ATPL notes and I had written 400ft - 1000ft so the 400ft was just a minimum!
Got it! thanks

[Spooky 2]

Intersting discussion. Someone correct me if I'm wrong, but I think the screen height for a wet runway is 35' and 50' feet for a dry runway. This is why you can see a max allowable T.O. weight higher on a wet runway than a dry one when using the Boeing Lap Top Tool or the IPT fuction on the B777 EFB.:}

[Mad (Flt) Scientist]

Intersting discussion. Someone correct me if I'm wrong, but I think the screen height for a wet runway is 35' and 50' feet for a dry runway. This is why you can see a max allowable T.O. weight higher on a wet runway than a dry one when using the Boeing Lap Top Tool or the IPT fuction on the B777 EFB.:}

Per the current regs, it's 35ft dry and 15ft wet:

§ 25.113 Takeoff distance and takeoff run.

(a) Takeoff distance on a dry runway is the greater of—

(1) The horizontal distance along the takeoff path from the start of the takeoff to the point at which the airplane is 35 feet above the takeoff surface, determined under §25.111 for a dry runway; or

(2) 115 percent of the horizontal distance along the takeoff path, with all engines operating, from the start of the takeoff to the point at which the airplane is 35 feet above the takeoff surface, as determined by a procedure consistent with §25.111.

(b) Takeoff distance on a wet runway is the greater of—

(1) The takeoff distance on a dry runway determined in accordance with paragraph (a) of this section; or

(2) The horizontal distance along the takeoff path from the start of the takeoff to the point at which the airplane is 15 feet above the takeoff surface, achieved in a manner consistent with the achievement of V2before reaching 35 feet above the takeoff surface, determined under §25.111 for a wet runway.

[Spooky 2]

Thank you!

[ECENE]

I guess 400' comes from those being the minimum OCH in the aerodrome area within a radius of X miles.
Don't have a copy of ICAO Doc.8168 right at hand, could anyone check that out?

JM2PW...

cheers!

[rockamora]

3.3 % climb gradient,394 feet & 15 degree turn !

does it ring a bell ? :O

[safeflyer]

As far as I remember, 400ft AGL is because 300 feet of obstacle clearance (The same that circling approaches). Because you can't think no obstacle exist (a house for example) 300ft + Obstacle (not high because of runway heading) rounded 100ft above will give you 400 ft. That is the minimum height to perform action safely in the very close vicinity of your airport.
If you need to accelerate because of engine failure, your accelerate height can be circling approaches minimum if VMC or HSD if IMC.

[4dogs]

One of the wonderful things that came about as a result of the ICAO Convention of 1944 was the consultative groups that were formed to develop the Standards and Recommended Practices for world aviation as empowered under Article 37 of the Convention. Those consultative groups not only thrashed out rules within their own areas of expertise but also ensured that the world was provided with one of the most cohesive and consistent set of guidelines for everything aviation.

400 feet appears in many of the ICAO Annexes - it is a standard level for obstruction charting, aircraft performance, instrument approach design, obstacle limitation surfaces for aerodromes, etc. There would have been many inputs into the original derivation of the 400 ft from all of those areas and undoubtedly many conflicting imperatives. I have no access to the historical data that would record whether the FAA adopted the ICAO figures or vice versa, but suffice it to say that 400 ft is a universal standard for control of the design of aircraft and the associated infrastructure.

ICAO is as much about an economic level playing field as it is about safety - lowering the minimum acceleration height would change to power requirements for aircraft (less), perhaps reducing other related performance requirements while substantially increasing the costs of rewriting all the AFMs, conducting airport surveys, making charts, installing obstruction lighting, maintaining obstacle control, limiting noise pollution and other environmental consequences, etc. And of course, each Contracting State would need to ratify and codefy the changes to an agreed schedule and....

I for one do not want less installed power or engines with shorter power ratings limits nor do I want to accelerate closer to the terrain than what is already a litle daunting if you watch the oibstacles coming at 1.6%.

[JammedStab]

We seem to use 600 feet on the airports we fly to(Boeing 727). Isn't it true that lowering the acceleration height increases the payload capability?

[DFC]

The 400ft above the runway and the 1500ft above the runway are as was said earlier simply markers to set performance values against.

They are specified in the certification requirements of FAR -23 and FAR - 25 as well as CS-23 and CS-25.

They ensure that certain standards are met.

For example a CS-25 twin engine aircraft must have a certain minimum climb gradient in the second segment. 400ft is simply the minimum height that the second segment can end at.

That prevents some manufacturer from trying to certify an aircraft that they claim has a second segment climb gradient of the minimum required but only up to 200ft!!!!!

Just like the balked landing minimum climb gardient requirement these are simply certification requirements.

Since the 400ft above the runway is a certification requirement there is no relationship to obstacle clearance which is a totally separate issue.

Edit to say that what is also tied in with the 400ft lowest end of the second segment is the 5 minute take-off power limit (10 minutes on some aircraft).

Without this 400ft yardstick, some designer could claim that an aircraft which could only manage the minimum climb gradient to 100ft and which had to level at 50ft in order to accelerate to min clean within the time limit when at max weight should be put into production!!

Regards,

DFC