Crash in Alaska and Lack of Probing About Key Safety System

It is not good when crash investigators reveal a distinct lack of curiosity. Case in point: the investigation into the crash in Alaska which killed the pilot, Sen. Ted Stevens and three other passengers. Four other passengers survived, although injured. (See Aviation Safety Journal, January 2011, “For Lack of a Locking Screw, a Crashed Airplane Could Not be Found Quickly”)

The DHC-3T

The DHC-3T

The National Transportation Safety Board (NTSB) just recently wrapped up its investigation into the August 2010 crash. The NTSB determined that the pilot, who had a history of stroke but had been granted a first class medical certificate after the event by the Federal Aviation Administration (FAA), was “temporarily unresponsive” as the airplane veered left into the path of high terrain.

The accident flight compared to the route that should have been taken

The accident flight compared to the route that should have been taken

The radar altimeter sounded a warning about 5 seconds before impact, and the airplane struck the tree tops in a climbing, left bank attitude indicating that the pilot was reacting at the last moment to avoid the terrain.

Left float, showing crush from the front

Left float, showing crush from the front

The airplane was equipped with a Terrain Awareness and Warning System (TAWS). This piece of avionics equipment could have alerted the pilot to dangerous terrain ahead. TAWS features a “look ahead” function that provides both aural and visual warning of looming terrain which is as high or higher than the airplane. This safety technology has saved many a pilot and his passengers from driving a perfectly good airplane into the ground.

Terrain altitude/color correlation on the TAWS display

Terrain altitude/color correlation on the TAWS display

But in this case, the TAWS was inhibited. In this mode, the aural and visual alerts of terrain ahead are deactivated. The pilot deactivates the system by pushing a button on the control panel. Investigators dug through the wreckage and found the TAWS control panel caked in mud. When the dirt was scraped away, the TAWS inhibit button was found in the depressed position – meaning TAWS essentially had been disabled by the pilot.

TAWS pushbutton found in the depressed (inhibited) position after the mud was cleared away

TAWS pushbutton found in the depressed (inhibited) position after the mud was cleared away

Investigators intimated that inhibiting TAWS is standard practice among many pilots in Alaska because of the system’s tendency to issue distracting nuisance alerts. These are not false alarms, but bona fide alerts based on the airplane’s height above terrain.

As NTSB Chairman Deborah Hersman stated:

“While aviation, especially general aviation, is a big part of life in Alaska, the risks of flying in Alaska are greater than in the continental U.S. There is unforgiving terrain – 39 mountain ranges with high peaks and deep gorges, and more than 100,000 glaciers. Then, there’s the challenging and rapidly changing weather conditions. Lastly, there are uncontrolled airports, dirt strips, lakes and rivers that serve as regular landing spots.”

One Board Member, Robert Sumwalt, was even more direct: “It makes no sense to me that to fly in Alaska you have to inhibit TAWS” [to reduce nuisance alerts].

The accident airplane was a de Havilland DHC-3T equipped with floats for take-offs and landings in the myriad lakes in the region. Lakes are not officially designated airports in the TAWS data base, so the system will alert the pilot when he is about to land on a lake, as he intends.

To suppress such an alert, TAWS can be inhibited. However, that can be done moments before landing. On the accident airplane, TAWS was inhibited during the cruise portion of flight.

Contrary to flights the previous days from the fishing camp on Lake Nerka southeast 52 miles to a remote fishing camp on the Nushagak River, the accident flight veered left to an east-northeast direction. The course change took the aircraft into mountainous terrain.

Accident flight path in red compared to flight paths with the same pilot on three previous days

Accident flight path in red compared to flight paths with the same pilot on three previous days

Had TAWS not been inhibited, the system would have issued an alert, “Caution, Terrain” about 30 seconds before impact. About 15 seconds before striking terrain the system would have sounded, “Terrain, Terrain, Pull Up, Pull Up.” The electronic map display associated with TAWS would have shown terrain 100 feet to 1,000 feet below the aircraft in yellow; terrain within 100 feet of the airplane’s altitude or higher would have been depicted in red.

What the pilot would have seen on the terrain display, had it not been inhibited -- no ground in black (a safe 1,000 feet below); rather, all yellow or red

What the pilot would have seen on the terrain display, had it not been inhibited -- no ground in black (a safe 1,000 feet below); rather, all yellow or red

With 30 seconds notice, a pilot should have had ample time to maneuver his airplane and avoid impact with the ground. That is, if TAWS is not inhibited.

A warning pop-up message, which would have been accompanied by a voice alert

A warning pop-up message, which would have been accompanied by a voice alert

Investigators were unable to determine why the pilot deviated from his previous routes and turned east-northeast. Did he have another stroke? Three autopsies were unable to find such evidence.

Investigators interviewed the senior pilot and fellow pilots at General Communications, Inc. (GCI), the owner/operator of the de Havilland float plane. NTSB investigators did not ask a single one of them about any habits on their part or the accident pilot to inhibit TAWS. Nor were other pilots in the region, flying for different companies, asked about any tendency to inhibit TAWS.

If pilots are inhibiting TAWS to suppress alerts of threatening terrain, maybe they are flying too low. After all, the accident pilot was flying about 100 feet higher than on previous flights through the mountain pass (where he suddenly turned left towards what a fellow pilot described as “smack in the biggest portion of the Muklung hills”). But the accident pilot was still flying lower than the tops of the hills.

Are there other cases in which pilots in Alaska are flying lower than the conditions warrant, with TAWS inhibited? Who knows? The records of interviews with other GCI pilots reveal no curiosity whatsoever on the part of NTSB investigators about these critical questions.

There were no recommendations from the NTSB to the FAA to find out if there is a widespread habit in Alaska for pilots to fly with TAWS inhibited – which is like flying without TAWS at all.

Any accident which occurs because a key safety system is inhibited or shut off goes beyond ironic tragedy. It is the very essence of a useless crash. There will likely be another because the NTSB did not inquire further.