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On airliners, should spoilers and reverse thrust be deployed before nose gear touches down?

I used to thought that deploying them will affect the handling characteristics, so it is best to wait until all wheels are on the ground; but I've seen pilots do it both ways. Does it depend on landing distance and runway condition?

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  • $\begingroup$ The main purpose of a spoiler is to "dump" the lift. e.g. a spoiler disrupts the airflow around the wing such that the wings does not produce lift in the spoilered area. Ideally you don't want do deploy the spoilers if you would like to roll the aircraft (depends on location of the spoilers). So you deploy them as soon as both rear wheels are on the ground. $\endgroup$ – Brilsmurfffje Oct 10 '16 at 8:35
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It depends on the aircraft (and conditions). In some aircraft like the MD-11, the deployment of ground spoilers causes a nose up pitching moment- in such a case, the spoilers should be deployed after nose landing gear touches down.

An example is the incident in Hawaii where the aircraft, a MD-11 exhibited a pitch up tendency during landing and the pilot had to initiate a go around. The factual report noted:

The MD-11 Flight Crew Operating Manual, Volume II, Landing Roll Procedure, contains the following note:

Ground spoiler deployment causes nose up pitching moment. This effect is most noticeable at aft centers of gravity. It is important to check the nose up pitching tendency with forward pressure on the control column and smoothly lower the nose wheel to the runway.

In some cases the deployment of thrust reversers could also have similar effect. In such cases, the thrust reversers and spoilers should be deployed after the nose wheel touches the ground, while in other cases, it is done after the main landing gear touches down and before the nose gear touches down. From FAA Airplane Flying Handbook:

... some airplanes tend to pitch noseup when reverse is selected on landing and this effect, particularly when combined with the noseup pitch effect from the spoilers, can cause the airplane to leave the ground again momentarily. On these types, the airplane must be firmly on the ground with the nosewheel down, before reverse is selected.

Other types of airplanes have no change in pitch, and reverse idle may be selected after the main gear is down and before the nosewheel is down.

There are other issues as well- deployment of thrust reversers before the nose wheel touches the ground means the pilot may not have sufficient authority for steering the aircraft in case one of them doesn't deploy. To prevent this some aircraft restrict the use of full reverse thrust till the nose landing gear is on ground (though idle reverse is permitted once the main gear touches down).

Also, for aircraft having engines in rear, the deployment of clamshell type thrust reversers can have clearance and debris ingestion issues which can limit their deployment. Also, the effect of spoiler deployment on the aircraft aerodynamics has also to be considered. For example, Boeing states:

The MD-80 and MD-90 in-flight spoiler lockout mechanism prevents the undesirable flight control configuration of deployed spoilers in either the speed brake or ground spoiler mode when flaps are extended.

There are also other aircraft (like Tu-154) where the thrust reversers were deployed before touchdown. So, all in all, it is better to consult the particular aircraft manual to decide when the spoilers/thrust reversers are to be engaged.

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    $\begingroup$ The MD-11 has auto spoiler deployment on wheel spin up, so the warning about pitch up is an advisory only and not meant to delay deployment after touchdown. $\endgroup$ – Mike Sowsun Oct 10 '16 at 12:04
  • $\begingroup$ See en.wikipedia.org/wiki/Air_Canada_Flight_621 for what happens when there is confusion about arming/deploying spoilers $\endgroup$ – DJohnM Oct 10 '16 at 21:32
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Spoilers

Spoilers are designed to be automatically deployed upon main wheel touchdown. There is no pilot selection unless they forget to arm the system or it is unserviceable.

Delaying spoiler deployment risks bouncing and reduced braking, as spoilers put weight on the wheels.

Some aircraft like the MD-11 with a relatively high-aft center of mass, have a two-stage auto-spoiler deployment, so variations will depend on the airliner. No pilot input is needed if the system is operative.


Reverse Thrust

Reverse selection is manually controlled and there will be some variation. Many pilots like to select reverse prior to nose wheel touchdown because reverse is more effective at higher speeds.

The MD-11 also delays the full deployment of the number 2 engine reverse, again no pilot input is needed, the pilot just selects the reverse on all three engines, and the system automatically takes care of the rest.

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  • $\begingroup$ It was posted as a wiki answer :) was it unintentional? If yes, then I didn't mean to intrude :) $\endgroup$ – ymb1 Oct 10 '16 at 21:21
  • $\begingroup$ I didn't realize I posted it that way. I answered using my phone and got mixed up. $\endgroup$ – Mike Sowsun Oct 10 '16 at 22:09

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