11
$\begingroup$

What would happen if a jet (say, a A330) were to touch down on the Engineered Materials Arrestor System (EMAS) just before the runway, instead of on it?

According to the FAA the EMAS is designed to "reliably and predictably crush under the weight of an aircraft". Will an aircraft touching down with its main landing gear crush and sink through this material as it would if it hit the EMAS after trying to stop on the runway or will the plane's momentum carry it on to the main part of the runway unharmed?

$\endgroup$
9
$\begingroup$

You'll probably sink in (and have a bad day.)

The longer answer is that it depends on how you land. Force from a touchdown on a runway can vary from nearly zero (in an extremely light landing) all the way up to twice the weight of the aircraft or more (in a hard landing.)

While it's theoretically possible to land an airplane very lightly on the runway, this normally requires a high forward speed and a very low vertical speed. It's rather unlikely that an incident resulting in landing short of the threshold (on the EMAS) would be at such a low vertical speed and high forward speed. The opposite situation (low forward speed, high vertical speed compared to a normal touchdown) is more likely in a situation of touching down short of the runway, otherwise you'd have probably made the runway.

As a result of this, the kind of landing that is likely to result in a touchdown short of the runway will result in forces that are minimally a significant fraction of the aircraft's weight and quite possibly at or significantly above that weight. Since the EMAS is guaranteed to buckle at less than the aircraft's weight in an overrun situation, it would almost certainly do so in the situation of landing on it.

The momentum of the airplane (specifically, the downward component of its momentum) will actually make it more likely for it to sink into the EMAS on touchdown, not less.

Of course, EMAS isn't designed for anything near the touchdown speed of an airliner. According to FAA Advisory Circular 150/5220-22B, a standard EMAS is designed to stop an aircraft that overruns the runway at 70 knots or less. A landing airliner will be flying much faster than that (Peter's answer here says 160 knots for a 747, for example.) This means that the backwards acceleration applied to the airplane by the EMAS will probably exceed the design forces by quite a lot, making for a quite uncomfortable landing for the passengers and quite likely breaking things on the landing gear (if not just breaking the gear completely off.)

The answer from Peter that I linked above also mentions that, in a hard landing where no flare is performed, the average force applied to the runway during touchdown would be just under twice the weight of the aircraft in the case of a 747, so that helps to provide a rough idea of the upper bound of the vertical forces imparted on the surface by an airliner at touchdown relative to the aircraft's normal weight.

$\endgroup$
  • 4
    $\begingroup$ If the aircraft sank in and then reached the end of the EMAS (i.e. the start of the runway), it'd probably shear the gear right off, since there would be no ramp to get the gear up out of the EMAS & onto the regular pavement, and still a lot of momentum on the fuselage itself. Bad day. $\endgroup$ – Ralph J Apr 19 '17 at 0:18
  • 1
    $\begingroup$ @RalphJ Good point. And that's assuming the gear hasn't already sheared off from running through the EMAS at more than twice the designed speed. $\endgroup$ – reirab Apr 19 '17 at 3:58
  • 2
    $\begingroup$ The FAA. Advisory Circular on EMAS states: "The EMAS must not cause control problems for aircraft undershoots which touch down in the EMAS bed." It shouldn't cause all the problems and damage you describe. $\endgroup$ – TomMcW Apr 19 '17 at 20:42
  • 2
    $\begingroup$ @TomMcW It seems to just say it won't cause you to lose control, not that it won't damage the gear and/or engine nacelles. Interestingly, it uses that requirement to specify a minimum strength for the EMAS material, which admittedly seems a bit counter-intuitive. I am now curious, though, how broadly the FAA defines "cause control problems." I'd think the inability to accelerate forward would be a "control problem," for example, but that seems to be a rather necessary consequence of traveling through EMAS. Maybe they mean only directional control? $\endgroup$ – reirab Apr 19 '17 at 20:59
  • 3
    $\begingroup$ They specify that simulator testing is sufficient. Not sure how you could really do they with a simulator. I'm sure broken landing gear would suffice for loss of control. Intuitively it seems like it would be a major problem, but if it would turn a landing short incident into a crash they would have gone back to the drawing board. It's intended for entry at 70 kn. My guess is that it's not all that deep, so if you're going 100+ kn you'd slosh right through it and there's probably a ramp up to the hard surface $\endgroup$ – TomMcW Apr 19 '17 at 21:10
9
$\begingroup$

will the plane's momentum carry it on to the main part of the runway unharmed?

An aircraft landing on the EMAS must not suffer control problems.


According to an EMASMAX document

FAA Advisory Circular (AC) 150/5220-22A
...

  • No adverse aircraft affects in the event of short landing ...

The referenced document doesn't state this but does say that EMAS should be set back from the runway end and

must not cause control problems for aircraft undershoots which touch down in the EMAS bed.


According to RUNWAY SAFETY AREA PROJECT - IGOR I. SIKORSKY MEMORIAL AIRPORT, there is only a three inch reveal at the transition of EMAS bed to runway. So a landing gear transitioning this part this part would likely not be catastrophic. I imagine crushed EMAS material still has some thickness.

enter image description here

$\endgroup$
  • $\begingroup$ Wow, great find, RGB! I spent far too much time looking through that PDF of the EMAS installation. $\endgroup$ – FreeMan Apr 20 '17 at 17:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.