More to it than meets the eye…
Both the FAA and NTSB tend to suddenly discover things that have long been a factor and make a big deal out of them. One or more accidents usually gets this ball in motion.
The latest hot button, from the NTSB, is what they choose to call tailwind landings. In what could have been a deadly serious accident, but wasn’t, an American Airlines 737 went off the end of the runway at Kingston, Jamaica. There were injuries but no fatalities. The 737 was pretty broken up about the matter.
You guessed it, the crew was landing with a tailwind.
The runway in use was 8,911 feet long. It was wet and there was a 14 knot tailwind component. The airplane touched down about 4,000 feet down the runway and the crew was unable to stop it in the remaining runway. It went through a fence, crossed a road, and stopped on the sand dunes and rocks just above the Caribbean Sea.
The regulations do not require any training on tailwind landings, nor does American Airlines provide any such training for its pilots. The airline does require that pilots perform an arrival landing distance assessment and that was not done by this crew.
I have studied other airline overrun accidents as well as similar general aviation accidents and there are always common threads. Too much speed at touchdown, whether from a tailwind or poor airspeed management, is almost always there along with wet surfaces and a touchdown well beyond the touchdown zone which starts 500 feet from the runway threshold.
As an aside, many (or even most) pilots do not fully understand the significance of runway markings. Those big thick white stripes that start 1,000 feet down the runway are aiming points, not touchdown zone markings. The touchdown zone markings start 500 feet from the threshold.
The American Airlines Flight Manual defines the desired touchdown point as within the first 800 to 1,500 feet beyond the landing threshold.
Clearly, if a tailwind landing is going to be attempted in any airplane, consideration has to be given to all factors.
When approaching my base at Hagerstown, Maryland, from the west I would request a straight-in to Runway 9 as long as the tailwind didn’t exceed 10 knots. The runway was uphill and the tailwind landing didn’t result in an abnormally long roll. The pilot’s operating handbook specified that it was okay to land with up to 10 knots of tailwind.
The airplane did feel different landing with a tailwind and I never managed smooth touchdowns though all were acceptable. I suspect that all airplanes are at least a little squirrely when landed with a tailwind.
The pilot’s operating handbook for your airplane should address the maximum acceptable tailwind for landing and give a method for calculating the amount of extra runway that will be required. On American 737s the maximum allowable tailwind is 16 knots. Southwest says 10 knots for the 737; five if the runway is contaminated.
There is one major factor in a tailwind landing that never seems to excite the curiosity of the investigators. It could have had a direct bearing on why this 737 crew wasn’t able to touch down until 4,000 feet of runway had passed beneath the airplane.
In theory the only increase in landing distance should be whatever is required to lose the additional groundspeed created by the tailwind. That would assume the same airspeed and altitude over the threshold and at touchdown as on an upwind landing. That, though, can be easier said than done.
The wind is almost always substantially stronger at, say, 2,000 feet than at the surface. At times it is quite a bit stronger.
For an example, say the wind at 2,000 feet is at 34 knots compared with the surface wind at 14, both acting as tailwind components for the approach and landing.
For the sake of argument let’s say the desired approach airspeed is 125 knots. At 2,000 feet the groundspeed would be 159 knots (125 plus 34 for the tailwind). If everything worked perfectly, the touchdown groundspeed would be 139 knots.
The twenty knot difference in the groundspeeds means the airplane would have to decelerate that much as it descends into the changing wind. This is easier done in a light airplane than in a heavier one, but it is something that a pilot has to think through ahead of time.
While decelerating on approach is something that had been done in heavy airplanes in USAF operations, it is probably not normally done in airline simulator training. There are even pilots out there who don’t actually believe this could be a factor.
A decreasing tailwind will, though, result in an increase in airspeed as the airplanes descends and if no accommodation is made for this, you might well land 4,000 feet down the runway. For this to happen you either have to be higher than the threshold crossing height or at an airspeed greater than Vref and with a contaminated runway either of those events should be cause for a go-around.
Back before we had GPS, a pilot had to use a little weather wisdom to anticipate this. There had to be some idea of the expected wind at 2,000 feet and the surface wind. Now there is nothing to it. Look at the groundspeed on the GPS while flying at 2,000 feet. If it is twenty knots greater than the airspeed, then you know how much you will have to decelerate while descending.
That changing wind with altitude is wind shear. Some pilots think of this only in connection with thunderstorms and there have been serious airline accidents caused by the failure of pilots to deal with storm-related shear. But a benign wind shear can also cause trouble and if a pilot doesn’t understand and anticipate this, he might find himself wondering what is going on with the speed on an approach. Wonderment is not one of the better sensations in a cockpit.