# How does thrust reverser selection have a destabilising effect on the rudder and increase the lateral skidding tendency of the aircraft?

I was reading in the FCTM landing section normal procedures the below paragraph.

The reversers have a destabilizing effect on the airflow around the rudder and thus decrease the efficiency of the rudder. Furthermore they create a side force, in case of a remaining crab angle, which increases the lateral skidding tendency of the aircraft.

Could someone please explain as to how this effects airflow over the rudder and increase the lateral skidding tendency of the aircraft?

• I don't have time to write a proper answer now, but tf there is a crab angle, the reverse trust is not aligned with the runway centerline. This angle causes the a sideways component of the thrust. – ROIMaison Feb 1 '18 at 13:05
• Interesting reading about rudder blanking on the MD80. Runway Excursion During Landing Delta Air Lines Flight 1086 Boeing MD - 88, N909DL New York, New York March 5, 2015 ntsb.gov/investigations/AccidentReports/Reports/AAR1602.pdf – Sports Racer Feb 1 '18 at 14:43

This is called rudder blanking. While I was searching for it, I came across this image

(Source)

As the Skybrary article mentions

Rudder blanking is a particular risk to aircraft with rear mounted engines and "T" tails.

I emphasized the word particular since I perceive it as an indication that it's a risk for other planes as well without rear mounted engines.

Now the flow from trust reversers, disturbs the airflow around the rudder reducing its effectiveness. In strong crosswind you rely on rudder to keep the aircraft centered on the runway. If you apply thrust reversers right after touchdown, you reduce the effectiveness of the rudder and in turn it might reduce the crab angle making it harder to keep the aircraft centered.

I will try to explain with a graphic, though as I've already stated in my comment, I'm not an expert.

As you can see in this image taken from Wikipedia, the thrust reverser output is not blocked by the wing, so it's quite safe to assume there is a jet similar to the first image.

I would estimate that the area of that vertical jet is more or less contained in the red circle seen in the next image. The free stream will push the turbulent jet from the reverser towards the back (as you can see in the first picture) and disturb the flow around the fin. The orange area is parallel to the free stream and shows the direction of the turbulent flow which might not be exactly the same in reality as the fluid particles have a tendency not to follow straight lines.

The plane is rotated 15 degrees to simulate a moderate "crab angle". From the picture posted in GdD's answer, you can assume this can be more if the vertical wind component is high enough.

(Source of original image)

Now one thing you might need to consider is that the engines of a wing mounted engine plane have significantly more distance from the vertical stabilizer compared to the tail mounted engine plane. That distance might be able to weaken the effect of the stream, enough to not cause accidents like this mentioned by Sports Racer in his comment. But that doesn't mean the problem is not there, hence the warning from Airbus.

• any reference for if rudder blanking affects wing mounted engines as I can't find any ? – Jai Feb 2 '18 at 5:39
• @Jai No unfortunately not; only my remark about particular in Skybrary that gives a hint about it. I could try to explain with a schematic, though it's not possible now and I'm not an aerodynamicist so I don't know if that would be accurate enough. – Stelios Adamantidis Feb 2 '18 at 9:10
• @Jai attempted to explain; hope it helps – Stelios Adamantidis Feb 7 '18 at 18:02
• your answer is correct I confirmed this with my company thank you for going through the effort also. – Jai Feb 8 '18 at 4:32

On a crosswind approach many pilots use the rudder to yaw into the direction of the wind in order to remain on centerline, which is what's called crabbing. In the picture below you can see that the aircraft is yawed (i.e. crabbing) to the right. The difference between the centerline and the airplane's heading is called the crab angle.

If a pilot maintains the crab to touchdown, which is a common technique, the aircraft will have a residual crab angle, like this.

If the pilot were to deploy reversers in this situation there will be a significant sideways force in the opposite direction of the crab, in the case of the picture towards the right, which could cause the airplane to skid in that direction.

• could you explain what is this sideways force? Also how does it have a destabilizing effect of the airflow over the rudder? – Jai Jan 30 '18 at 10:41
• @GdD consider the relative airflow in your above picture, and you'll see that you don't have any sideslip angle. This means no rudder is required to "remain on centerline" as long as you have an appropriate crab angle. The rudder is not really used until when you wish to decrab (just before or after touchdown, depending on aircraft and environmental conditions). – Waked Jan 30 '18 at 11:37
• I'm not attempting to answer the rudder part of the question @Waked, just the side component to the thrust reversing. – GdD Jan 30 '18 at 11:56
• @GdD I do understand that, and I do not mean to nitpick, but I thought I'd let you know, as you still might want your entire answer to be factually correct. – Waked Jan 30 '18 at 14:46
• I'm happy to refine it when I have some time @Waked, what is it you don't understand? – GdD Jan 30 '18 at 14:47