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I just came across the Wiki article on Lauda Air Flight 004, which was destroyed when a thrust reverser activated in mid-air.

In the article, it says:

The incident led Boeing to modify the thrust reverser system to prevent similar occurrences by adding sync-locks, which prevent the thrust reversers from deploying when the main landing gear truck tilt angle is not at the ground position.

But that can't possibly be enough. Landing gear is lowered and locked while the plane is still flying. That means these "sync-locks" would be released while the plane is still in the air.

Edit: Looks like I misunderstood what "not at the ground position" meant. Apparently the truck tilt angle is not flush except when the wheels have pressed onto the ground. Nevertheless, the general question of what safety mechs there are, still stands.

Are there any other safety mechanisms? Are there any that detect, somehow, that the wheels have actually touched down? Maybe a wheel RPM sensor?

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    $\begingroup$ I'm not sure if this a dupe, but there's a lot of overlap with these two questions: here, here $\endgroup$
    – Pondlife
    May 26, 2017 at 17:41
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    $\begingroup$ I'm just speculating from the text, but a 767's landing gear seems to be at an angle when it's in the air (see this image for an example of what I mean). So maybe the text is saying that the sync-lock looks at not just whether the gear is down, but its actual angle -- the assumption being that it will only be 90° (or whatever) when the airplane is actually on the ground, pushing down on it. $\endgroup$
    – yshavit
    May 26, 2017 at 17:58
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    $\begingroup$ Many aircraft have a WOW (weight on wheels) switch. It's commonly mounted on the oleo strut such that when the oleo is compressed, the switch is closed. $\endgroup$
    – Simon
    May 26, 2017 at 18:14
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    $\begingroup$ The truck tilt angle is not in the "ground position" when the gear is lowered. If you look at a landing aircraft, you can see that the trucks (the axles the wheels attach to) are at an angle. See this image for an example. $\endgroup$
    – Ron Beyer
    May 26, 2017 at 19:12
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    $\begingroup$ Safety is often a matter of tradeoffs. Lufthansa flight 2904 (an Airbus A320-200) had better safeguards against accidental deployment of the thrust reversers, which contributed to it overshooting the runway: Computer logic prevented the activation of both ground spoilers and thrust reversers until a minimum compression load of at least 6.3 tons was sensed on each main landing gear strut, thus preventing the crew from achieving any braking action by the two systems before this condition was met. $\endgroup$
    – chirlu
    May 27, 2017 at 12:59

2 Answers 2

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On a Boeing 777, the thrust levers need to be at their idle stops otherwise the thrust reverse levers physically cannot be moved aft. Once the thrust levers are at their idle stops and the reverse levers are moved to the reverse thrust position there will be three signals provided to the thrust revereser actuation system (TRAS).

These three signals to the TRAS energize the thrust reverser sync lock valve (SLV), the directional control valve (DCV), and the isolation valve (IV). At a reverse thrust lever angle of 11 degrees, the SLV is energized by closing a microswitch in the thrust lever assembly. The SLV then sends hydraulic pressure to the sync lock on each thrust reverser sleeve to accomplish release.

At 24 degrees reverse thrust lever angle, the DCV is energized by a microswitch in the control column assembly where the thrust levers and reverse thrust levers reside. The valve is shuttled by system pressure to the deploy position.

At thrust resolver angle of <31 degrees the IV is opened by a signal from the electronic engine control (EEC), and system pressure, by way of the DCV, is ported to the actuators. The IV command, from the EEC, also requires the engine to be running and the airplane to be on the ground.

Air/ground logic uses radio altimeter signals, with ground mode for the thrust reversers being < 5 feet. The locks within the locking actuators release and the thrust reverser sleeves move aft. After the thrust reverser sleeves move aft by a measure of 60%, interlock releases and the levers can then be moved to their maximum reverse thrust position.

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The important bit in your wikipedia quote is

main landing gear truck tilt angle

as the gear trucks hang at an specific angle when not on ground. Take a look at this picture of a B767-300ER (same a/c type). You can clearly see the gear truck tilted forward.

It is not hard to imagine that the gear truck will be parallel to the fuselage after touch-down. Only then the reversers can be unlocked. When the gear is unloaded (aka the aircraft flies) the gear truck will take its forward tilted position, thereby inhibiting the reversers.

I don't know the exact engineering solution on the B767 but it is most likely the same as for the B747: a simple reed switch which closes when the gear truck is tilted at the correct angle. The switch is mounted on the gear strut and the magnetic target on the gear truck.

Truck tilts -> target comes close to the switch -> switch closes -> aircraft must be in the air -> no reversers for you.

EDIT: After you refined your question: Generally speaking there are different ways of sensing if the aircraft is on ground. All have their shortcomings. Here are some:

  • gear tilt (as explained above)
  • oleo strut compression (when the gear strut is compressed there must be weight on it, therefore the a/c must be on the ground)
  • radio altitude (when the radio altimeter senses zero or negative values the a/c must be on the ground)

The wheel RPM you suggested is not used - when you land on a wet runway there is a good chance you'll end up hydroplaning down the runway with load on the gear but no wheel rotation.

Airspeed is not used as well. Reverser deployment short final would be disastrous. And tying it to the stall speed (Vs) would be counterproductive as you touch down with a speed well away from Vs.

Hope this helps,

Chris

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