Crew missed trouble signs before 737 crash, investigators say

In the minutes before a Turkish Airlines Boeing 737-800 crashed last week in Amsterdam, the plane's crew apparently missed a series of indications that a crucial instrument reading was wrong and that the plane was slowing dangerously.

The Dutch Safety Board investigating the crash, which killed nine people, including three Boeing engineers, said Wednesday in a preliminary report that one of two altimeters, which measure the plane's altitude upon approach, was faulty.

Because of the altimeter's false reading, for 100 crucial seconds the plane's autothrottle drastically cut power to the engines as the airplane descended.

The pilots may not have reacted adequately to that instrument failure.

In a notice sent Wednesday to airlines, Boeing listed a half-dozen warning signs that can alert a pilot that something is wrong with the altimeter.

The Dutch report said Boeing should strengthen its warning in the 737 manual that pilots shouldn't use the automatic landing systems when there's a malfunction of the altimeter.

In response, Boeing issued to airlines "a reminder to ... carefully monitor primary flight instruments during critical phases of flight."

"We're saying, 'Pay attention,' " Boeing spokesman Jim Proulx said.

One worked, one failed

The Dutch report said the jet's left radio altimeter was providing a faulty reading, while the radio altimeter on the right side gave a correct reading. But the pilot-side instrument on the left, unless overridden by the crew, is the one that feeds data to the plane's automated systems, Boeing said.

That faulty signal told the airplane's automatic landing systems that the jet was much lower than it actually was — an impossible altitude of negative 7 or 8 feet, when the airplane in fact was still 1,950 feet above the ground. Low cloud cover was reported at the site, so the crew probably couldn't see the ground as they began their descent.

The incorrect altimeter reading prompted the plane's automatic landing systems to cut the engine power to idle and raised the nose slightly to prepare for touchdown, the report said.

For nearly two minutes from that moment, the engines had minimum power.

"Initially, the crew did not react to the issues at hand," the Dutch report said.

The speed of the airplane fell well below the proper approach speed, and the jet dropped beneath the glide path required to reach the runway. At about 700 feet, the plane broke through the clouds to perfect visibility on a calm, dry morning.

Yet, the pilots took no action until a stall warning buzzed and shook the control column at just 490 feet above the ground.

Full power then was applied and the pilot brought up the airplane's nose, but it was too late.

Impact at 108 mph

The plane fell from the sky, striking tail-first at 108 mph, then whipping into the earth with the greatest force at the front of the airplane.

The crew and five passengers died, including the three Boeing engineers from the Puget Sound area, who were traveling in business class up front. A fourth local Boeing engineer was badly injured and is still hospitalized.

Three pilots were in the cockpit that day. Capt. Hasan Tahsin Arisan was experienced. The first officer in the right seat, Olgay Ozgur, was in training.

A third pilot in the jump seat behind them, Murat Sezer, was there to monitor the pilot in training. His presence on the flight deck indicates that the first officer had less than 25 hours' experience in flying this size of jet, according to a pilot who spoke on condition of anonymity.

Trainee at controls?

The Web site of trade magazine Flight International, citing an unnamed source, reports the trainee was at the controls and that as the airplane lost height, the captain was talking him through the landing checklist.

Boeing's listing of warning indicators that should alert pilots to an altimeter malfunction includes:

... The discrepancy between the two altimeters. The first officer and captain are supposed to cross-check their independent readings.

... A warning, both visual and with a buzzing sound, that the landing gear is not down — even though the airplane is still flying high. The Turkish Airlines crew received this warning, the voice recorder indicates, but didn't recognize it as a problem.

... The persistent display during descent of a blue "RETARD" warning light, meaning the throttle is at idle. If the throttles go to idle during a descent, the display is supposed to flash from "RETARD" to "ARM" after a few moments, to prompt pilot action.

An Alaska Airlines 737 pilot who asked not to be named said the Dutch report suggests the faulty altimeter started a sequence of events to which the pilots reacted too late. Pilots have to be aware that instruments sometimes fail, he said, and they must know how to detect and handle that.

"In a low-visibility landing, with any kind of input that says things are not normal, typically that crew should [abort the landing] and figure out what's going on," the pilot said. "These airplanes are landing at 175 miles per hour, and you can be down to 600 feet of visibility. At that speed, there's not much room for error."

The black box of the Turkish Airlines plane provided investigators with data covering its past eight flights. The data showed that the faulty altimeter problem had occurred twice previously in similar situations, just before landing.

It's unclear if there was any documentation of those incidents by the airline, or if they were even noticed at the time.

Dominic Gates: 206-464-2963 or dgates@seattletimes.com