Originally Posted by Centaurus
Air Rabbit. On a slippery runway it is obvious that wheel braking efficiency can be greatly reduced. The 737NG FCTM discusses reverse thrust and crosswinds on a slippery runway. From reading that you simply have to be careful when using reverse but it certainly does not imply you should not use reverse. If you start sliding sideways the FCTM advises reduction to idle reverse until straightening with brakes then re-applying reverse. With no crosswind and symmetrical reverse on a slippery runway it will be the effect of reverse at high speeds that will kill the speed and allow more effective use of anti-skid. The key thing is to apply full reverse as soon as possible after touch-down where the reverse is most effective.
I am not saying that reverse thrust should not be used when landing on a slippery runway.
Quite the contrary. And I completely agree that use of reverse at the higher speeds provides the greatest effect. Reverse thrust is a significant aid in slowing the aircraft, no matter the conditions. However, what I AM saying is that the use of reverse thrust is inappropriate for
directional control,
particularly on a slippery runway. And I think the language you quoted from the Boeing manual confirms that position.
It is my strong belief (and my practice in both operations and training) that when using reverse thrust to slow after landing on a slippery runway, and a drift or a yaw is encountered, the pilot should immediately reduce the amount of reverse thrust being used - all the way to idle reverse if necessary - until directional control is regained by either aerodynamic control, differential braking, nosewheel or rudder pedal steering, or a combination. If directional control is not regained quickly after going to idle reverse and applying the other techniques, the pilot should not hesitate to select forward idle thrust and continue to attempt regaining directional control through these other means. Continuing to attempt to regain directional control through the use of reverse thrust, even differential reverse thrust, will almost always only exacerbate the problem, and could well assist in the aircraft departing the runway edge.
Drifting toward the side of a runway after landing is due either to the directional momentum of the airplane just prior to touchdown or because of a crosswind. Either situation is made more complex by the existence of a slippery runway. Physics is always involved and it becomes significant when the direction of the thrust being used is anything other than parallel with the runway centerline. It is true that differential thrust (forward or reverse) will tend to input a pivotal moment around the center of mass; and the effectiveness of that input will be dependent directly on the coefficient of friction between the runway and the tires – the lower the coefficient of friction, the greater the effectiveness of the rotational moment. Unfortunately (or probably fortunately) none of us are ever trained on the amount of asymmetrical thrust necessary to bring an aircraft back into a “parallel-with-the-centerline” position, much less, to do it precisely. Remember the old saying, “anything that will take you to it, will take you through it?”
However, because we’re talking about putting an airplane on a surface that is a whole bunch longer than it is wide, and keeping it on that surface, we have to recognize that
any directional moment (from either thrust or wind) is divided between the axis parallel with the centerline and the axis perpendicular to the centerline. It is the perpendicular axis that becomes critical. Because of the relative narrowness of runways, it is always desirable to NOT have a directional moment along that axis at any time. The fact is that, with the airplane centerline not parallel with the runway centerline, OR should the airplane centerline become other than parallel with the runway centerline (for any reason), any thrust moment or any crosswind will produce a moment along that axis.
The aircraft can be positioned in the center of the runway, on the upwind side of the runway, or the downwind side; and the aircraft can be aligned “parallel” with the centerline, or aligned “nose upwind” or “nose downwind.” Obviously, on the centerline and parallel is the desired condition. Anything other than having zero momentum toward either side of the runway and either a zero wind, or a wind directly down the runway centerline, will result in a tendency for a lateral axis movement. In the more likely event you have either an “off-centerline momentum and/or a cross wind, and unless you have a pretty good amount of “into-the-crosswind” momentum (which would tend to be dampened by the effect of that crosswind) you are more than likely to wind up on the downwind side of the runway. Once on the surface, the amount of movement will depend on the coefficient of friction to mechanically control the direction (brakes and steering), and the ability to control the airplane direction aerodynamically. As the coefficient of friction goes down, so will the ability to control the airplane mechanically, and the effect of any off-centerline momentum or crosswind component will be increased.
If you are, indeed, on the downwind side of the runway, there are 3 potentials for the direction of the aircraft nose: parallel with the centerline, nose downwind, or nose upwind. We would have generally three scenarios: the “parallel” position is likely to become “nose upwind” (due to weathervane) and moving closer to the runway edge (due to the direction of the wind). The “nose downwind” is likely to become “parallel” first, continuing on to “nose upwind” (also due to weathervane) and moving much closer to the runway edge, because of the time involved to move from “nose downwind” to “nose upwind” and for a portion of time the off-axis forward idle thrust is helping move the airplane closer to the edge. However, the “nose upwind” position will not weathervane (at least not much more so, as it is already pointed into the wind – or mostly anyway), and the tendency for the wind to further blow the aircraft toward the edge would be countered (at least to some extent) by the forward idle thrust.
If the off-axis momentum was downwind and at fault for getting to the downwind side of the runway in the first place, then the forward idle thrust will have an effect on adding or neutralizing the tendency to drift further downwind and toward the edge as follows: a neutral effect, then a positive effect on the “parallel” situation; a negative, then neutral, then positive effect on the “nose downwind” situation; and a positive effect on the “nose upwind” situation.
If you now add the factor of selecting reverse thrust after touching down – after all the above circumstances have started to operate – the result would be as follows: The “nose downwind” situation would initially be “negative” (forward thrust pushing you toward the runway edge) – and as the airplane weathervanes into the wind, the forward thrust value (that would have gone to positive) will now go to negative, as reverse thrust is selected. The “parallel” situation would not initially change as forward thrust is exchanged for negative thrust. However, as the aircraft weathervanes into the wind, the positive value of the forward thrust, now becomes negative with the reverse thrust. The “nose upwind” situation would negatively change as forward thrust is exchanged for reverse thrust.
Significantly, in any of these situations, the entire time the engines are in reverse thrust, the effect is pulling the aircraft toward the runway edge. If asymmetric reverse thrust is used – yes, there will be a tendency to impart a rotational moment – and its effectiveness is directly related to the coefficient of friction. And, in the mean time, the additional higher reverse thrust from that engine or engines will only bring you closer and closer to the runway edge.
I still advocate the use of reverse thrust after landing – even on slippery runways. It is only when directional control becomes an issue that I have an issue with the continuance of reverse thrust – at least go to idle reverse – and be prepared to go to forward idle thrust. Just like the Boeing manual says.
Sorry for the long-winded response.