If a mere sliver of metal can bring down a jet, Heathrow's runway patrol have some of the toughest jobs in aviation. Tim Walker meets the crew who keep us up in the air

Mark Devonald squints at a tiny, grey speck in the giant, grey sky and identifies it as a 747.

He spotted military planes for fun as a boy; now he spots commercial planes for a living, as an airside duty manager at Heathrow Airport. At this particular moment, however, he's somewhat preoccupied with a different sort of bird: a crow, perching on a runway marker, just a little too close to the runway for comfort as the Jumbo makes its final approach. Part of Devonald's job is to make sure that real birds don't come into contact with metal ones. "You get to learn that every species has its own personality, just like the planes," he says. "And crows are right bastards."

From the flock of Canada geese that collided with a US Airways Airbus A320 causing it to ditch in the Hudson River last January, to the 43cm titanium strip that struck Concorde's tyre on take-off from Charles de Gaulle airport near Paris – setting off a chain reaction that resulted in a crash, 113 deaths and a court case only now coming to a climax 10 years later – the smallest thing can create a huge headache for the aviation industry. And needles like that titanium strip don't come in haystacks any bigger than Heathrow, which services more international passengers than any other airport in the world – over 67 million of them each year, on almost half a million aircraft.

The ripple effect of closing just one of the airport's pair of runways is felt within minutes: the "stack" of aircraft in a holding pattern above Heathrow starts to build up, which quickly impacts the rest of UK airspace, then European airspace, before holding up all those flights headed for Britain's biggest port (Heathrow also welcomes 1.3m tonnes of air cargo annually) from the other side of the Atlantic. Soon, there are angry tourists sitting on their suitcases all over the evening news, and talk of the likely economic impact.

At the frontline of this perpetual battle for punctuality is the airport's Airside Operations Safety Unit (with the unmemorable acronym, AOSU), of which Devonald is a member. AOSU's empire stretches from the departures lounge to the perimeter fence, as well as the air in a 13km radius around the airfield. The responsibilities of the 130 staff under the leadership of each shift's duty manager include: remotely allocating and directing each flight to one of the 230 parking stands that serve the five terminals (and, frequently, marshalling the planes with the famous orange paddles when they get there); overseeing up to 50 sets of building contractors who are at work on Heathrow at any given moment, maintaining the miles of runways, taxiways and their 20,000 light fittings; and dealing with emergencies large and small. Today, Devonald has to race out to the runway with the airport fire crew to receive a Lufthansa jet that's landing with a minor electrical fault. In the end, the plane lands normally without assistance; this sort of thing happens once or twice daily, he explains. Driving very fast seems to be the most thrilling part of an AOSU operative's day. They speed along the runway between take-offs and landings, picking up "Foreign Object Debris" (FOD), anything from chunks of rubber to suitcase handles to – and this really happened – handfuls of okra. They are helped by high-tech "FOD radar", a system designed specifically to seek out small objects on the runway, and installed in 2008, after the Concorde crash.

A challenge for the airside team in recent weeks has been the heavy snowfall. AOSU has a large team of snow clearers and de-icers ready to drive a collection of advanced mechanical contraptions out onto the runways and get them "back to black" in double time. And yet, says Colin Wood, the airside operations director, one of the most difficult months last year was July, "when strong winds had just as much effect on the flying programme as the snow".

Wood took charge of airside in 2006. A fortnight into his reign he welcomed the very first Airbus A380 to the airfield, a commercial airliner so large that Heathrow was forced to widen its taxiways, build new aircraft stands and passenger lounges, even adjust the radar settings to accommodate it. Since then he has endured the gruelling opening of Terminal Five, overseen the smooth switchover to a brand new control tower – so smooth, indeed, that it was barely reported – and dealt with the aftermath of a crash, when a British Airways flight landed short of the runway in January 2008. There were only minor injuries, and within an hour of the incident the runway was open again. "The aviation industry is very good at sharing its experiences," says Wood, "so I've been all over the world to talk to other airports about what I learned from that crash."

He recalls: "I was on the way back to my offices when I got the call to say something had happened... By the time I got to the site, 45 minutes later, the passengers were on buses heading to the terminal. The cabin crew did a remarkable job, and so did our team." Wood had experience with such emergencies from his previous career, as an air-traffic controller in the RAF. "After I left the military, I told my wife I'd take a less stressful job," he says. "Then I came to Heathrow."