Mr Butt, 51, said there were plenty of things he would miss after today, such as the daily walk from his office to the hangar to greet his pilots, aircrew and aircraft engineers in an environment that is kept meticulously clean and organised, with every tiny item neatly stored at arm's length. The hangar is his favourite place, where he enjoys chatting and sharing the latest experiences of his pilots and their crews.

When the GFS was set up in April 1993, it represented arguably the first solid step by the then colonial government to demonstrate that Hong Kong was really to be returned to China in a few years. The GFS took over the duties of its predecessor, the Royal Hong Kong Auxiliary Air Force (RHKAAF).

Mr Butt joined the RHKAAF in 1986 as its first local full-time pilot. Previously, he had been a police chief inspector specialising in the detection of counterfeit banknotes.

'When I was young, I dreamed of working as a policeman or a pilot. I learned how to fly when I was a student in Alberta, Canada. After graduation, I went back to Hong Kong to get a job. Because I had insufficient flying hours I could not get work with an airline and so I applied to join the police force, and became an inspector in 1977. In the meantime, I volunteered as a pilot for RHKAAF.

'My big opportunity came in 1986, when the RHKAAF head asked if I was interested in taking up a full-time pilot's job. I jumped at the chance, since my passion was - and still is - aviation, and I've been here ever since.'

In 1993, the RHKAAF was disbanded and GFS was formed to make way for the 1997 handover. Three years later Mr Butt was promoted to the top job as its controller and tasked with implementing a series of measures to ensure its smooth transition the following year.

'My utmost priority was to ensure that safety procedures and professional standards did not deteriorate,' he said. 'I am very happy to say that the GFS has maintained the highest standards, and it soon proved its professionalism when local pilots and crewmen performed their first offshore night rescue. I thought the complete transition could take anything up to a decade to achieve, but within only four years we attained the highest standards.'

Localisation of the staff was another key development for the service. During his first five years as controller, Mr Butt gradually introduced more native Hongkongers to the team, and today the staff of 206 is entirely Hong Kong-born.

'To me it's like a big family and I am a proud father,' Mr Butt said. 'Unlike other professionals, a GFS pilot spends a lifetime learning and testing. For instance, every 12 months every pilot and crewman must test their physical fitness and skills. People need passion and endurance to go through our tough training as well.'

He said his worst moment for the GFS came in August 2003, when a helicopter on a routine medical evacuation mission crashed into the side of a hill on Lantau a few minutes after taking off to pick up a sick person on Cheung Chau. Both the pilot and crewman were killed. Black box recordings showed no sign of equipment failure.

'If anybody dies on my watch, no matter what happened, I am ultimately responsible for it. I will take the memory of this crash to my grave,' he said.

But he also recalled his proudest moment, last year, when 20 of his fliers received bravery medals at Government House for risking their lives rescuing people in danger during Typhoon Prapiroon in 2006. He recounted how on August 3 that year, an alarm at the GFS Air Command and Control Centre was raised, marking the start of a long-range search and rescue mission. A group of mainland crew members from two barges were in distress off Shangchuan Dao, about 172 km southwest of Hong Kong. During the ensuing 24 hours, four missions were flown and 91 people plucked from storm-ravaged seas.

'For long-range offshore rescues, two pilots and two crewmen are deployed, and they risk their lives to rescue people in dire peril. You must trust their skills and the reliability of their equipment. To see those young aircrew whom we recruited and have trained return safely from a difficult rescue mission is my happiest moment.'

Mr Butt has always made it a point not to be desk-bound. 'Yes, I am forced to spend much of my time at my desk, but I have always tried to be an operational pilot,' said the controller, who has logged 6,000 hours of flying fixed-wing aircraft and helicopters. 'It's a matter of balance. I can't be in the way of my junior pilots, who often need more experience and flying hours. I often have had to step aside to let them do the job.'

Mr Butt has also overseen a continuous upgrade of the GFS fleet. 'When I joined, we had a range of only 100 nautical miles [185km] for sea searches in daylight,' he said. 'Now we are able to do night missions covering the South China Sea up to 700 nautical miles south of Hong Kong, although most of our operations take place within a range of 400 nautical miles.'

The fleet comprises seven helicopters and two fixed-wing aircraft, none more than 10 years old. The introduction of the sophisticated Super Puma L2 helicopter was a particular boost to the fleet. The all-weather machine has advanced equipment such as a night vision-compatible cockpit, special search radar and a belly-mounted fire tank for firefighting.

On average, GFS helicopters fly 4,000 to 5,000 hours a year and fixed-wing aircraft 1,000 to 2,000. The number of callouts for emergencies was 2,534 in 2005-06 and 2,447 in 2006-07. For 2007-08 up to March 19, the figure was 2,125. Mr Butt is pleased that search and rescue callouts for careless hikers have dropped from 583 in 2005-06 and 473 in 2006-07 to 393 for 2007-08.

'Despite this we are constantly called out for emergency fire-fighting work because of the carelessness and selfish people,' said Mr Butt, 'especially during traditional festivals such as Ching Ming and Chung Yeung.'

'To curb abuses from ill-prepared hikers, we came up with a new measure 18 months ago and so far it has worked well. We bring a mountain rescue team from the Civil Aid Service or the Fire Services Department to the scene, then winch them down to assess the case. If it's not an emergency and hikers can cope with the situation, we leave the rescue unit to walk the hikers down the hill safely. One result of this is that we're getting very few abuses these days.'

There has been much speculation about why Mr Butt is leaving early, especially after other senior staff have retired or resigned in the past few years.

'It's about time and I've done enough,' he said. 'We have a sustainable succession plan. With the job market booming, it's no surprise to see staff turnover. I'm confident about the GFS' future because of the working conditions, harmony and the nature of the job. Any pilot will find it more satisfactory to rescue people.

'But if one day I am on an airliner flown by one of my former colleagues, I'll be proud to tell the passengers sitting near me that he's from the GFS.'

After retirement, Mr Butt intends to remain involved with aviation on the mainland with a flight training school.

'Hopefully I can raise flying standards and safety there with my international perspective on aviation. I will continue to fly, but not in Hong Kong - it's far too expensive for a retired civil servant,' he said.

Asked how he planned to celebrate his last day in uniformed government service, Mr Butt cast his eyes over the tarmac of Chek Lap Kok and said: 'I'll call the air command centre to put me on the standby list. If there is any callout, that would be my last flight.'