Landing gear extension.

[retard-retard]

Hi all,

Is there a written rule at what point in the approach the landing gear should be extended or is it airline SOPs specific?

Cheers.

Mark.

[Reds Blues Greens]

It is airline SOP specific.

I have used a couple of SOP's and in both cases the gear is selected at a specific point in relation to glideslope intercept; namely a dot and a half fly-up deflection.

You will notice that at most airports when you look back up the approach at the approaching aircraft, that they may appear levelled-off for some distance before beginning their descent down the glideslope. This is known as the level-off or platform altitude. As the aircraft approaches the glideslop signal, flying level, the fly-up deflection will get less and less; upon reaching one and a half dots (two being maximum) the gear is usually selected.

The gear can be selected earlier if determined necessary and safe by the flight crew. This may be required on 'hot' approach where the aircraft is both high and fast and requires more drag.

Or the gear maybe selected later where a high speed approach is required e.g. 160 knots to 4 dme, and selecting gear in the normal position would just result in more power being required uneccessarily.

I haven't explained that very well :ugh:

Can anyone else help?

RBG

[retard-retard]

You have explained it just fine RBG, no problem.

Would you know if the dot and a half fly up is also specific to BAW?

Thanks.

Mark.

[Reds Blues Greens]

Sorry Mark, what is BAW? :confused:

[retard-retard]

British Airways ;)

[Reds Blues Greens]

Yes, I believe it is also the SOP used by BA. Although in many major airports due to high speed approaches the gear will not be selected at that point.

Also a dot and half represents an angular displacement from the glideslope and on some higher platform altitudes the gear maybe selected later, such as one dot (and vice verca).

Cheers,

RBG

[Vee One...Rotate]

Hello,

Hopefully a related quick question - don't want to hijack the thread.

How many feet does each "dot" equate to? I presume it isn't a/c specific. On a related note, how many degrees does each dot along the localiser needle's travel equate to?

Cheers!

V1R

[Reds Blues Greens]

There is two dots of deflection in each direction for both the localiser and the glideslope indicators. The displacment that is diplayed on either of these is a function of angular displacement.

For a localiser full scale deflection = 2.5 degrees

For a glideslope full scale deflection = 0.7 degrees

For any given angular displacement e.g half-scale on localiser (1.25 degrees) this will represent less and less distance from the centre line (or zero deflection on the loc) the closer to the transmitter that you get.

The the ILS is an incredibly sensitive system. There is a statistic for the size of the box you are trying fly the aircraft through to maintain half scale deflection (for both loc and gs) at a certain distance; but it escapes me now.

On a instrument with five dots full scale (typically in light aircraft) flying just one degree of a radial when tracking a VOR would give half a dot deflection whilst if you were flying an ILS the same deviation would give nearly half scale deflection! The legal limit for continuing the approach.

RBG.

[BOAC]

RBG's reply is correct as far as a 'classic', training or 'textbook' approach goes for many airlines. As he has indicated, in the real world, one rarely gets to fly such an approach.

'Gear down' can be required at many stages of an approach. The earliest I have expoerienced is 80 miles from landing due to a need to reduce landing weight. Putting it down at 10000 ft or so is not unknown, particularly when a sudden change in landing runway (probably due to the national carrier wanting a 'straight-in approach' on the runway 'not-in-use' and a change to a sudden 'calm' wind from the original 10 knots down the runway:D :D ) leaves one incredibly high on the approach profile and needing extra drag. Personally I try to announce the forthcoming event to the salary payers if time permits so as to avoid any undue anxiety.

Most airlines specify a 'minimum' height for gear extension, commonly 1500' if visual with the runway or 2000' if not. Other than that, it is down to necessity/preference.

[Vee One...Rotate]

Thanks RBG,

V1R

[TotalBeginner]

What about during a continuous descent approach?

[Fright Level]

Although we tend to use the glideslope dot as a gear down point in the sim and at airfields with a low platform height (2,500 feet), BA's SOP's actually call for the aircraft to be in a stable landing configuration by 1,000 feet. On the 747 we'd normally lower the gear somewhere between 2,500 and 2,000 feet to give it time to lock, then select landing flap (you can't select landing flap without the gear locked down without getting a pesky warning).

This timing generally means you're all configured (and cross checked) well ahead of the SOP requirement at 1,000 feet AAL.

[retard-retard]

Thanks EDDNR,

are you aware of BA's SOPs on the Airbusses as these (as I'm wholly sure you're aware) have a different flap config. than their Boeing counterparts?

Cheers.

Mark.

[Right Way Up]

On an ILS or good weather non-precision approach, start configuring to be stable at 1000ft AGL. On a 737/A320 that will mean taking gear at 1500 ft latest, or 747 approx 2000 ft. This really does cut down on the noise that the neighbourhood has to suffer. If bad weather non precision approach i.e. ceiling below FAF alt, then be fully configured by descent from platform altitude so you can concentrate on the approach, and reduce the amount of variables.

[Easy226]

Right way up,

You have made an interesting point there as I have always wondered why aircraft are louder when the visibility is poor.

I live about 10 miles out from Birmingham and noticed that it is noisier when the vis./weather is poor - I was always under the impression that the clouds were to blame for the noise?