ATSB Sends Garuda Recorders To Manufacturer

Air Safety Week, March 19, 2007

Investigation Into Fatal Crash Continues

Even though the Flight Data Recorder (FDR) was apparently more damaged than the cockpit voice recorder (CVR), only the FDR has yielded information on the events leading up to the Garuda 737-400 crash on March 7 that killed 22 people of the 140 aboard.

Australian Transport Safety Bureau (ATSB) Deputy Director Joe Hattley said: "We have tried every method we can to download the cockpit voice recorder, but without success and that includes in-depth consultation with the component manufacturer Honeywell in the U.S.".

However, raw data yielded by the FDR had been sent to the Indonesian KNKT (NTSB equivalent) without any analysis. It is also understood that only a limited number of parameters were recoverable. The bureau's executive director, Kym Bills, said it was up to Indonesian investigators to assess the data decoded so far. "It's more of a matter of checking it against the physical evidence and the other evidence that they've gathered on the accident site and in relation to the whole investigation," Bills said.

In that Garuda Flight GA200 accident, passenger and witness descriptions of their perceived sequence of events indicates that the nosewheel was "driven in" in a potentially catastrophic nosewheel first touchdown off a very fast ("hot") approach, and that a destructive Pilot-Induced Oscillation (PIO) called "porpoising" then began. Porpoising can become a divergent phugoid in a jet that's landed far too fast for its weight (and with insufficient drag flap).

What's Porpoising?

After the nosewheel strikes first, its oleo rebounds upwards and the aircraft maingear "bounces" (courtesy of the MLG oleos compressing and then decompressing a short time later). The neophyte pilot's natural tendency is to recover by lowering the nose and again to "spot the deck" (i.e., try to force the airplane onto the ground). It's completely opposite to a normal flare and hold-off process. The inevitable result is another nosewheel first strike (and maingear rebound). The PIO is underway.

This apparently happened three times and on the third occasion the nosewheel oleo snapped and the wheels departed. Complicating matters in turboprops and piston-engined airplanes' PIOs is the instant power response that's available. Pilots can easily get "out of sync" by adding power in the bounce (i.e., on the rebound). That added power cycling tends to "eat up" runway remaining and stops the desired airspeed bleed-off.

However, to achieve the same porpoising effect in a jet, you just have to be "hot" (and high) over the threshold, have little or no flap (drag to kill off your float speed), and try to force the airplane onto the ground (resulting in a tricycle landing or even worse, striking nosewheel first). The "eat-up of runway remaining" complication can be provided in a jet by another system (see below) and by the lack of flaps and early spoiler and reverser deployment.

Adding to these self-inflicted woes is the fact that any power "adds" (or throttle manipulation) will inhibit spoiler extension and add to the overrun likelihood. Even without throttle jockeying, you still need the MLG squat switches to be depressed long enough for the spoiler panels to pop up (and then of course, the oleos to remain depressed).

During porpoising, that just won't happen. Lacking that spoiler effect, the wing just keeps "flying". Porpoising is a PIO cycle that's destined to end up destructive and/or off the end, particularly if you're hot and have landed too far in. We can expect to see many more of these types of "mishandled, confused and fixated" accidents the world over, as pilot experience levels drop and new pilots with Multi-crew Pilot Licences (MPLs) move up in the airline world.

This theory may explain the underlying cause for Flight GA200's fate once they'd touched down. But why did they allow the aircraft to get so "hot" (i.e., fast) on finals? It may be that:

a. The RH seater just neglected to select flap (or misplaced the lever at <15[degrees] and then totally forgot about the flap lever because of the distraction of an overshoot trend) or,

b. The flap speed relief valve (aka load limiter) operated because of the high speed (regardless, flaps should extend normally to the selection once speed is reduced). Flaps can be operated hydraulically or via a backup system, electrically, as long as they are operable and not locked out, or

c. The aircraft suffered a flap lockout, either due to asymmetry (flaps extend a little but an asymmetry trip-switch cuts off the hydraulics to stop a greater asymmetry developing) or, for lockouts occurring in the flaps 0 to 25 range, suspect structural interference, drive-gearing failure, jammed transmissions, foreign object damage, or hydraulic system failure as being the most probable causes.

The EGPWS will give a call of "Too Low Flaps" at 250ft AGL if flaps aren't at 30[degrees] or more. This is cancellable via the Ground Proximity Flap Override Switch. This is what likely happened: an unnoticed lockout.