We were going to have "a perfectly normal landing", assured the pilot of Lufthansa flight 499 from Mexico City to Frankfurt, as he prepared his wary passengers for an emergency landing in the Azores. It was anything but.

"We have smoke in the cabin," he said."We cannot find out the reason. We will be landing in Lajes."

We were seven hours into our 11-hour voyage across the Atlantic.

The crew went into panic mode: one, in tears, wrenched down the curtains between sections of the cabin, lest they impede an evacuation. Video screens went blank, lights out, air-conditioning off.

The voice of the purser was shaking and breaking as she spluttered instructions: "Fasten your seat belts, ensure that your seats are ..."

Couples gripped hands across the aisles, mothers held children. Hardened flyers grasped armrests. An elderly Mexican woman crossed herself repeatedly.

A few wept, but some crew members wept harder.

"Do they know something we don't?" I asked the estimably sanguine German across the aisle from me. He had worked as an electronics engineer for Boeing, which had made this 747, and explained that the pilot would descend steeply to reduce the time during which - if there was a fire - flame could spread as air temperature increased. "If there's something serious, it becomes more serious when you're towards the ground," he advised, expertly. I wasn't sure I wanted to know. Down we went, the plane shaking, overhead baggage-lockers swaying, mumbling expletives and devotions, whether we believed them or not, every flyer's nightmare.

Earlier we had boarded on timeand an announcement came as we settled: a fault with the brakes, we'd have to wait two hours on the ground while it was repaired, extending time on board to a minimum 14 hours. There was an irony for me: I had opted for a flight via Frankfurt rather than fly direct with British Airways because my last trip with BA had been delayed by three ghastly hours on the tarmac in London while they fixed a tyre.

Indeed, close observers of air travel note how long delays for aircraft repair have become common, warning that as airlines cut costs they enter what John Stewart of AirportWatch calls "a grey area between savings and safety".

He said: "There is increasing regularity in these stories; airlines will say this is because there are more planes in the air, and there may be some truth in that. But they are under such pressure from fuel costs, not putting passengers up for the night and getting planes where they are supposed to be, they'll do anything not to incur extra costs. It gives passengers no comfort that they appear to put cost-cutting in favour of safety."

The cabin of LH499, going nowhere, grew intolerably hot.

The crew opened the emergency exits, but the rear left door was opened "in error", admitted the captain; an emergency slide had been forced, now this, too, would need repair. I went to the rear cabin to take some air through the broken door. It was a daunting sight: door hanging, innards of wiring all over the place, on which ground engineers worked by torchlight from a stairway.

Midnight passed, the door eventually fixed, but Lufthansa demanded 50 passengers seated nearest the door leave the aircraft, shuffling off. "Please note that the last door on the left side cannot be used in case of emergency," advised the purser. Tape was fixed across the aisles to seal off the rear.

The purser also noted: "It is still very warm in some parts of the aircraft," and a Scandinavian passenger, an executive for a heating and cooling company, worried about switching off the air-conditioning system.

At 01.50 Mexican time, after 5 hours on board, we began our long flight, us claustrophobics calculating an upcoming total of nearly 18 hours on board. Only that did not happen.

All flights had been cancelled at Lajes Airport in the Azores, due to high winds, but down we went. The smell of smoke reached mid-cabin; this was raw and real fear. "PLEASE have fixed the f***ing brake," someone shouted. We hit the ground, expertly - swayed, swerved and halted.

We sat there, told "to keep your seat belts fastened. The fire brigade is waiting to check that we don't have any more smoke".

The former Boeing engineer was aghast: "So they're checking for fire WHILE we sit on the plane!? First, we'll make sure there's no fire, then you can get off? Isn't this the wrong way round?"

A fire truck from a US air force base pulled up and men in silver space suits boarded. "The fire brigade has come on board to make sure there is no more smoke," said a superfluous announcement of cold comfort. And after a while: "We will be arranging hotels for you here." So out we went, into the humid, high winds of the Azores.

As we gathered in a terminal, waiting for explanation and news about our journey, Lufthansa's staff vanished. I called home to ask whether information could be gleaned by phone from the airline, but like everyone else was told by my partner that following a long wait she had been dismissed brusquely: "There's no information. He'll just have to catch the next flight."

At 6pm Azores time, 11pm next night in Mexico, we arrived at a hotel in the windswept, sea-swept (on any other occasion, lovely) mid-Atlantic port of Angra do Heroismo. Still no sign of Lufthansa, apart from a printout on the reception desk about a bus at 9.30am.

Next morning, the crew suddenly re-appeared at the doorways of a replacement plane from Germany, parked in front of the kaput flight 499.

The captain got straight to the business of departure, adding: "We apologise for what happened yesterday." But while our bags were transferred from The emergency landing of a nightmare flight across the Atlantic

499, a revolt in business class - I later learned - demanded an account of "what happened yesterday".

"There are some irritations," said the captain, "about the technical standard during the flight from Mexico. We did not take off in Mexico with a technical status in doubt. We took off from Mexico with a technically perfect aircraft.

"Later in the flight we had a situation. We had smoke in the cockpit, smoke in the front aisles. As a precautionary measure, we landed here on the Azores."

No information about what had caused the "smoke".

We arrived in Frankfurt, 27 hours late. "Thank you for choosing Lufthansa," said the captain. "I hope we'll see you on another Lufthansa flight some time, and that you have enjoyed your flight with us."

Last Tuesday came a callback from Lufthansa, to partly explain. Spokesman Klaus Gorny said the original problem had been a hydraulic leak, not in the braking system but landing gears, which "needed repair". It had been discovered during the captain's final "walkabout".

Moreover, "an emergency [door] slide had been de-activated, because the door had been opened by accident ... It was an error." The opening of the door was "for the delivery of additional food and beverages", and the 50 people removed from the aircraft "because we need to be sure that we have the right ratio of passengers to emergency doors".

Are you sure there's nothing about air-conditioning? Gorny consulted his brief: "Oh yes, when they started the repair to the aircraft, they shut down the air-conditioning. The emergency slide was opened and activated, for both reasons [heat and food]."

By Thursday the aircraft was again flying Mexico-Frankfurt, and two days later Lufthansa answered detailed questions. But on the cause of smoke, said Gorny: "Investigations are still in progress and we are therefore unable to provide further details."

However, "the status of the aircraft was technically flawless until the first fault appeared. There was no fire detected during the entire flight or on the ground after landing. Immediately after landing and with the help of infrared cameras, the fire brigade detected from the outside that there was no open fire on board. There was no reason to evacuate the aircraft and there was no danger to life on board at any time."

Stewart said: "Lufthansa should not have flown that plane. Clearly, so much had gone wrong with it before takeoff. It's like your child bumping their head badly at the seaside, and you drive home to see your usual doctor, when you should go to the local hospital."

Once we had all got home, traumatised, Lufthansa sent an email: "We are delighted that you chose to fly with Lufthansa on your last trip. Day after day we do our utmost to meet your needs with our services. We would be very grateful if you would take just a few minutes time to rate your last trip with LH499."

-Observer