EO Screen Height and LG Retraction

[Airmann]

During an Engine Out Take off it is necessary that the aircraft reaches the screen height at the end of the TODA. This is critical in order to meet obstacle clearance.

However, in my sim sessions I often see (and have been guilty myself) people requesting gear up before the aircraft has reached the screen height.

I've been thinking recently that this is a seriously unwise move, due to the added drag, and may result in the aircraft not reaching the screen height in time. In addition I believe performance calculations assume that the gear stays down until screen height.

Does anyone have any more information regarding this?

[oggers]

In addition I believe performance calculations assume that the gear stays down until screen height.

Performance calculations do not assume the gear stays down until screen height. Gear may be raised once the aircraft is airborne.

I've been thinking recently that this is a seriously unwise move, due to the added drag,

:confused: I don’t think raising the gear increases drag.......

[Cough]

:confused: I don’t think raising the gear increases drag.......

When you select the gear up on say a 744, how many doors open prior to the gear retracting? Those open doors cause drag, which is what oggers is referring to.

Airmann - The 787-9 has a 'pre doors' function, whereby the gear doors open upon liftoff, in order to reduce retraction time once the gear is actually selected up. That may well defeat what you may be trying to achieve.

[sonicbum]

I've been thinking recently that this is a seriously unwise move

And Your thoughts are that keeping Your eyes and the other pilots one on the radio altimeter till You see exactly 35 ft is wiser ?

[Jesse Pinkman]

Waiting till 35' isn't going to make the door drag go away. You've got to deal with it sometime and the sooner you can get rid of the gear drag, the sooner you can start making good on the profile. Sitting around waiting for the RADALT with the gear hanging out is worse than worthless.

[EI-PAUL]

During an Engine Out Take off it is necessary that the aircraft reaches the screen height at the end of the TODA. This is critical in order to meet obstacle clearance.

However, in my sim sessions I often see (and have been guilty myself) people requesting gear up before the aircraft has reached the screen height.

I've been thinking recently that this is a seriously unwise move, due to the added drag, and may result in the aircraft not reaching the screen height in time. In addition I believe performance calculations assume that the gear stays down until screen height.

Does anyone have any more information regarding this?


Hi Airmann.
I don't think that the performance calculation specifies that the gear should be down until reaching the screen height.
The screen height is the "reference zero" from which the first segment starts. The first segment, by definition, ends when the retraction of the landing gear is finished, and this is the only thing specified regarding this matter as I understand it.
If we think pragmatically, I do not believe that if the take-off performance calculation was correctly performed in relation to weight, environmental conditions, length and runway conditions there is a considerable risk NOT to meet obstacle clearence.
This is especially true when we perform a flex take-off, which generally leaves further room for performance.
The priority should be to stabilize the horizontal and vertical trajectory ASAP to avoid gross handling inaccuracies.
My 2 cents

[PEI_3721]

Considering the ‘big picture’, the effect of gear door drag, etc, might be less than inaccuracies in flying the aircraft after a ‘surprising’ failure and implementing an emergency procedure. This also applies to focus on raising the gear ASAP, opposed to delay or even forgetting due to ‘irrelevant’ considerations.

[john_tullamarine]

Further to PEI_3721 's post (and I note that he is a technical expert in these sorts of matters), the AFM will prescribe where the gear is to be selected up and the performance data will be predicated on this information. Your company Operations Manual information should reflect whatever the OEM had to say in the AFM.

Some of the posts suggest that some of the operators are not doing their bit by providing AFM guidance to line crews ...

[sheppey]

On the subject of gear retraction, the Boeing 737 FCTM states use the barometric altimeter only and not both the IVSI and baro altimeter indicating, as proof of positive rate of climb before gear retraction is initiated. Why is this so?

Commonly observed in simulator training is the PM calling "positive rate" when in fact the aircraft is barely airborne, and the PF calling "Gear Up" based on the PM's call rather than double checking his (the PF's) own altimeter is rising. This quirk is most often seen when the Non-Normal exercise is captain's static vent blocked before take off. In his case the F/O calls "Positive rate" on his own altimeter while the captain's altimeter and IVSI don't move due to the blocked static vent.

But the very action of calling "Positive Rate" by the F/O triggers the Pavlov's Dog reaction by the captain of answering "Gear Up" to the call - not to his instrument readings.

[vilas]

I don't think raising the gear below 35ft makes any difference. . Finally, the flight path must be based on the aircraft’s performance without ground effect. As a general rule, the aircraft is considered to be out of the ground effect, when it reaches a height equal to its wing span. Most commercial aircraft will be in ground effect at 35ft so that will add to the performance. Also the first segment OEI minimum required climb gradient is 0%.

[MarkerInbound]

First segment requires a positive climb gradient so .0000000000000000001 would be acceptable but I don’t think a zero gradient climb would work.

[vilas]

0% for twin And .5% for quad. Page 64, Getting To Grips with aircraft performance

[EI-PAUL]

0% for twin And .5% for quad. Page 64, Getting To Grips with aircraft performance


CS 25.121 Climb: one-engineinoperative (See AMC 25.121) (a) Take-off; landing gear extended. (See AMC 25.121(a)) In 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) and in the configuration used in CS 25.111 but without ground effect, the steady gradient of climb must be positive for two engined aeroplanes, and not less than 0·3 % for three-engined aeroplanes or 0·5 % for fourengined aeroplanes, at VLOF and with – (1) The critical engine inoperative and the remaining engines at the power or thrust available when retraction of the landing gear is begun in accordance with CS 25.111 unless there is a more critical power operating condition existing later along the flight path but before the point at which the landing gear is fully retracted (see AMC 25.121(a)(1)); and (2) The weight equal to the weight existing when retraction of the landing gear is begun determined under CS 25.111.

Ref EASA CS-25 Amendment 19

[hans brinker]

CS 25.121 Climb: one-engineinoperative (See AMC 25.121) (a) Take-off; landing gear extended. (See AMC 25.121(a)) In 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) and in the configuration used in CS 25.111 but without ground effect, the steady gradient of climb must be positive for two engined aeroplanes, and not less than 0·3 % for three-engined aeroplanes or 0·5 % for fourengined aeroplanes, at VLOF and with – (1) The critical engine inoperative and the remaining engines at the power or thrust available when retraction of the landing gear is begun in accordance with CS 25.111 unless there is a more critical power operating condition existing later along the flight path but before the point at which the landing gear is fully retracted (see AMC 25.121(a)(1)); and (2) The weight equal to the weight existing when retraction of the landing gear is begun determined under CS 25.111.

Ref EASA CS-25 Amendment 19
Well, he did say 0%, not negative 0%.... :)

[FlightDetent]

I'll re-phrase the question: Are there any specific provisions in the certification realm, how the adverse aerodynamic effect of L/G retraction is accounted for?

On a performance-limiting runway (think EFATO - "Go" on wet, TODA limited) around the screen height point, the margins inherent to the daily calculation are quite narrow.

claim no knowledge of how bad the degradation would be, though through hearsay an open NLG wheel well with the doors extended acting like endplates delivers quite a punch on the 737classic at high AoA. Any CDL guru to shed a light?

[john_tullamarine]

Are there any specific provisions in the certification realm, how the adverse aerodynamic effect of L/G retraction is accounted for?

Indeed. The FT program actually does retract the gear during performance takeoff testing and the protocols adopted are described in the AFM. Whatever effect(s) may be due to the gear's retraction sequence is captured by the FT data gathering process.