Herald file Searchers look for debris the day after the crash of Swissair Flight 111 on Sept. 2, 1998. All 229 people on board died.

By Stephen Thorne / The Canadian Press

Ottawa - Pieced together from cockpit recordings, control tower logs, black box data and radio communications, a timeline released on the eve of its second anniversary details the last moments of doomed Swissair Flight 111.

The three-page chronology, released by the Transportation Safety Board of Canada, sheds a little more light on what happened aboard the MD-11 jet in the minutes before it plummeted into the ocean off Peggys Cove, killing all 229 people aboard the night of Sept. 2, 1998.

But as Saturday's anniversary passes, some events detailed in the evolving timeline and previously released transcripts bring more questions than answers about those last terrifying moments aboard the plane.

For example, Boston regional air traffic controllers tried without success to raise Flight 111 for 13 minutes shortly after takeoff.

Was the plane's electrical system acting up already? Were the pilots on the wrong frequency or did they simply take 13 minutes to switch frequencies after controllers had cleared them to do so?

If Capt. Urs Zimmermann and co-pilot Stefan Loew were having electrical problems, they either weren't aware of them or simply said nothing about them when they re-established communications with Boston.

Outside of massive structural failure, electrical problems can be the most serious snag a pilot encounters. It's unthinkable Zimmermann and Loew would not have reported such a problem, experts say.

Besides, the black box, which records aircraft workings, shows no evidence of electrical problems that early in the flight - just 18 minutes out of New York's John F. Kennedy airport.

Records from every other controller in the area indicate the pair were not communicating with anyone on another frequency, investigators say.

In spite of the blackout, no one bothered to ask Flight 111 if there was a problem. The plane's electronic signature was bright and clear on radar screens, so controllers knew it was still aloft, approaching 27,000 feet.

Why there was a blackout may never be known.

The cockpit voice recorder ran in a half-hour loop. By the time the plane went down almost an hour later, there was no record left of what transpired between Zimmermann and Loew during that critical period.

It wasn't until 10:10 p.m. ADT - almost an hour into the flight - that the crew detected an unusual odour in the cockpit, but they did not report it right away.

The voice recorder indicates they checked with cabin crew and found no evidence of an odour aft of the cockpit.

They suspected it was a problem with the air-conditioning system.

A minute later, at 10:13 p.m., smoke was visible in the cockpit. By that time, the flight crew was communicating with regional air traffic control in Moncton.

"Swissair 111 heavy is declaring Pan Pan Pan," they declared at 10:14 p.m. as the plane passed over West Berlin, Queens County.

"We have smoke in the cockpit, request deviate, immediate right turn to a convenient place, I guess Boston."

Evidently, the pilots didn't realize the gravity of their situation. Pan is an international call sign indicating a problem that is not yet an emergency.

Heavy refers to a large, wide-body aircraft.

Less than a minute later, the pilots were wearing oxygen masks.

Moncton Centre suggested Halifax was a closer alternative for landing - 70 nautical miles as opposed to 300. Twenty-eight seconds passed before the pilots confirmed their preference was Halifax.

The plane was then cleared to descend to 10,000 feet.

At 10:17 p.m., the cabin crew was informed the flight was diverting to Halifax.

The plane was cleared to 3,000 feet, but Zimmermann wanted to level at 8,000 to prepare for landing. Moncton Centre told the pilots they could stop descending at whatever altitude they wanted.

Moncton advised: "You've got 30 miles to fly to the (runway) threshold."

Fourteen seconds later, the pilot responded: "We need more than 30 miles."

Moncton cleared the plane to turn north to lose altitude. At that point, the controller asked a telltale question and Zimmermann made a fateful decision.

"Swissair 111, when you have time could I have the number of souls on board and your fuel on board please for emergency services?"

Zimmermann responded: "Roger, at the time, fuel onboard is two-three-zero tonnes. We must dump some fuel. May we do that in this area during descent?"

Apparently there was confusion in the cockpit. The figure quoted - 230 tonnes - was the airplane's gross weight, not the amount of fuel onboard.

Fuel is generally dumped over water at altitudes of less than 10,000 feet.

"Are you able to take a turn back to the south or do you want to stay closer to the airport?" the controller asked.

"Standby short, standby short," came the response.

Twelve seconds passed.

"OK, we are able for a left or right turn toward the south to dump."

Moncton cleared the plane for a left turn, taking the lumbering aircraft west, away from the airport, and south over Peggys Cove.

"Advise me when you are ready to dump," the controller said. "It will be about 10 miles before you are off the coast. You are still within about 25 miles of the airport."

The pilot told Moncton the plane was descending to 10,000 feet to dump fuel. Moncton said it would inform the pilot when the plane was over water.

Zimmermann apparently clicked his microphone on by mistake. A mixture of Swiss, German and English crackled over the airwaves.

"Du bist i dr emergency checklist fur air conditioning smoke?"

"You are in the emergency checklist for air conditioning smoke?" the controller asked.

"Ah, sorry it was not for you," the pilot said. "Swissair 111 was asking internally."

The aircraft continued turning south at 10,000 feet. The time was 10:23 p.m.

Moncton told the plane it was 35 to 40 miles from Halifax airport if it had to get there "in a hurry."

Fourteen seconds later, the autopilot disconnected.

Over an apparent warning tone, the voice of the pilot came over the radio: "Swissair 111. At this time we must fly manually. Are we cleared to fly between 10 thou - 11,000 and niner thousand feet?"

A few seconds later, at 10:24 p.m., Swissair Flight 111 declared an emergency. Both pilots appeared to be talking at once:

"Swissair 111 heavy is declaring an emergency," said one.

". . . we are between 12 and 5,000 feet we are declaring an emergency now at (Greenwich) time zero one two four," said the other.

"Eleven heavy we (are) starting dump now we have to land immediate."

In the background, the autopilot disconnect warbler could be heard.

Four seconds later, the pilot repeated his plea: "And we are declaring an emergency now Swissair 111."

Eight seconds later, at 10:25:06, air traffic control lost altitude information from the plane.

"Swissair 111 you are cleared to commence your fuel dump on that track and advise me when the dump is complete."

There was no response.

"Swissair 111 check you're cleared to start the fuel dump."

Again, no response.

At 10:25:40, the black box stopped recording. A second later, the cockpit voice recorder shut down.

Ten seconds later, at 10:25:50, air traffic control regained altitude information from the doomed plane. Then, at 10:26:04, the plane vanished from radar screens. Its last recorded altitude was 9,700 feet.

Five minutes after that, at 10:31:21, the MD-11 jet slammed into the water.