Why 400ft AGL at Boeing aircrafts?

[Speedwinner]

Hey folks,


why does boeing require 400ftagl to activate lnav and actions on the engine failure?History?


cheers


SW

[Denti]

Depends on Model and options bought i guess. LNAV is active from 50ft AGL on our 737s, VNAV will be active from 400ft on and command the correct engine out schedule upon engine out detection, however since only part of our fleet is correctly equipped we cannot use VNAV armed before take off yet.

The 400' AGL for the first action probably is something left over from the min accel. height used during certification. But of course i could be totally wrong there ;)

[Mach E Avelli]

I think that the choice of 400 ft was based on the idea that it was more important to fly to the minimum acceleration allowed under most situations and thereby get some terrain clearance going before fiddling with FMS selections, thrust levers, fuel cutoffs and fire handles. As mentioned above, later models have the option of departing in LNAV but the idea of minimal changes below 400 ft seems like a sound enough scheme.

[bigduke6]

On 747-400 LNAV has been active at 50' for the 9 years that I have been flying it. And glad it is, when you consider most of the RNP departures out of Europe. No idea if it is a customer selectable option or not? Old NWA training stuff from years ago seems to be the same.

VNAV active at 400'. Recent company and FAA/POI change now lets us engage autopilot at 250' (on initial take-off only, not go-around/missed approach), verses waiting to 500' like we had been doing for years.

I have never seen a limitation altitude on doing engine failure memory items, but prudence would seem to dictate wait until a bit of altitude, like 400-500', get the A/P on, etc, instead of hurrying and doing them right at V2 and 50'. But if that is what you chose to do, and it works out, well, ok I guess............http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/icon25.gif

[shabo8]

Very good question...I often wondered that myself and thought maybe it had something to do with fact that all SIDs assumed no inadvertant turns below 400ft agl (Jeppesen Manual). However a relatively new SOP has been introduced at our company whereby the FO arms LNAV on the MCP on the below the line b4 T/O Chklist but there are some exceptions like BARAJAS, MADRID. Overall not sure myself.

[Phil Squares]

IIRC, the 400' height comes from the end of the second segment climb. Many, many years ago, I remember the airline I worked for had a 25 flap takeoff procedure on the 727. The reason given for the 400' flap retraction altitude was it was the end of second segment climb which is from 35' to 400' AGL with the flaps in the takeoff position.

That's my guess.

[Mad (Flt) Scientist]

Its somewhat related to 25.111(c)(4)
The aeroplane 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 aeroplane is 122 m (400 ft)
above the take-off surface;

Although that's a condition for takeoff performance, and not a limitation on pilot actions as a whole, it embodies the idea that up to 400ft, there should be no pilot actions required to achieve acceptable performance. An implication is that there should therefore be no pilot actions PERMITTED in that same segment of flight, for workload etc reasons. Certainly it's often difficult or impossible to take credit for any pilot actions below that height in assessing risks and failure scenarios.

[MD83FO]

aircraft is already plural isn't it? so many people writing "aircrafts" that I'm getting confused.

[mustafagander]

Way off topic but....

MD83FO, perhaps some posters are ESL??

Try your skills at posting on a, say, Swedish language BBS!!

[kilomikedelta]

Accuracy in communication is always in order and a priority. Lack of accuracy in communication (including computer languages) often results in loss of life. Aircraft can refer to one or a number of aeroplanes.

[411A]

Phil Squares


Correct, the 400 AAL requiremnt dates from CAR4B, predating 14CFR25.

[de facto]

Phil,

As far as i know, the second climb gradient doesnt start at 35ft but when gear is retracted.

Obviously 400ft Height is if there is no obstacles that would require a higher MFRA.

[Phil Squares]

I would argue, we're both wrong.

We both forgot the requirement for the aircraft to be out of ground effect. :ok:

[HazelNuts39]

As far as i know, the second climb gradient doesnt start at 35ft but when gear is retracted. or at 35 ft, whichever occurs later along the take-off path.

[de facto]

Hazelnut,

You are talking about take off distance..
I think in any scenario, the aircraft (737 in my case) will have passed the 35ft screen height by the time the gear is up.

No?

[bArt2]

why does boeing require 400ftagl to activate lnav and actions on the engine failure?History?


Getting safe ground-aircraft separation before getting heads-down could be a reason. The old rule "the minimum height to start a turn on departure is 400 feet" could have something to do with it.

However, on aircraft where LNAV can be armed on the ground, it will become active well before 400 feet AGL most of the times.

And finally the " I sue you " culture in the US could have something to do with it because it causes them to stick with procedures that have been around some time and have been proven to work.

Bart

[HazelNuts39]

de facto;

I'm talking about the take-off path, which comprises the take-off distance and the take-off flight path.

Whether the gear retraction is complete before or after 35 ft depends on the time required for the gear to retract and the climb performance with one engine inoperative. At a weight close to the WAT-limit gear retraction may be complete before 35 ft on some airplanes.

Just for precision, since you raised it.

regards,
HN39

[aterpster]

Mad (flt) Scientist:


Although that's a condition for takeoff performance, and not a limitation on pilot actions as a whole, it embodies the idea that up to 400ft, there should be no pilot actions required to achieve acceptable performance. An implication is that there should therefore be no pilot actions PERMITTED in that same segment of flight, for workload etc reasons. Certainly it's often difficult or impossible to take credit for any pilot actions below that height in assessing risks and failure scenarios.

And your predecent quote of Part 25-type language:

The aeroplane 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 aeroplane is 122 m (400 ft)
above the take-off surface;

Thus, the pilot is expected to retract the gear well prior to 400 feet; i.e., a pilot action to achieve acceptable performance.