A B737 pilot reported that the HUD unexpectedly extended to the full down position and could have hit the Captain's head. A double locking mechanism may prevent such an event.

Narrative

Twice in the last several years; I've had the HUD unexpectedly fall from the full up position to the full down position; just missing impact with my head. The first time the HUD dropped came after a firm landing. The second; most recent event occurred two days ago while performing a max thrust takeoff. In both cases; I probably did not raise the HUD up firmly enough to cause it to lock completely. However; I'm sure I'm not the only pilot ever to make that mistake. Should this HUD ever impact someone's skull; it could do some pretty good damage. I suggest that a double locking mechanism be installed to prevent the HUD from falling all the way down from a full up position. I think a second latch at the midpoint would be a good idea. This would require the pilot to manually unlock it twice on the way down from the up position. This would be a minor issue and be as easy as unlocking it from the up position. A huge benefit would be the protection provided the pilot in the case where it's raised enough to stay but not lock. With the second lock installed; the HUD would stop before head impact. Another solution would be rigging something to prevent the HUD from staying up if it isn't fully locked.