An interesting document from the Airport Authority crossed my desk the other day. It reveals a grubby little secret that third-runway boosters would love to keep dark.
It is dated July 12 and was written by the authority's deputy director of projects, Kevin Poole. It makes perfectly clear that the real reason our airport is congested is that the airlines find it convenient to operate much smaller aircraft there than they did at the old Kai Tak airport.
It means all those mainland tourists who we thought were bringing us so much money are actually going to cost us HK$130 billion in the construction of a third runway. The airlines make more money by flying them in on smaller aircraft.
Mr Poole has a talent for expressing his thoughts succinctly. On the principle that no knife is so sharp as the one on which you cut yourself, I shall thus quote him directly:
"Regarding traffic forecasts, it has been noted the growth in passenger demand included in HKIA Master Plan 2030 (MP2030) exceeds that of the design capacity as listed in the 1992 New Airport Master Plan (NAMP) by about 10%.
"This has led some to believe that the airport may not truly be reaching its saturation point, and that the excess capacity for flight movements will be used predominantly for private jets.
"The discrepancy between the forecasts is mainly because many of the working assumptions adopted in the early 1990s were based on the operating environment of Kai Tak Airport, which was highly constrained and fully stretched.
"At the time it was natural for airlines to maximise each valuable slot by deploying the biggest aircraft possible. The 1992 NAMP therefore assumed that wide-bodied aircraft would comprise over 80% of aircraft movements, resulting in a high average passenger load forecast of more than 300 people per aircraft.
"The new airport at Chek Lap Kok provided more runway capacity, allowing airlines to increase their flight frequencies and service to secondary destinations. This has enabled HKIA to develop into an international and regional aviation hub but it also led to the deployment of more narrow-bodied aircraft (mostly less than 200 seats).
"Since 2000, the average passenger load per aircraft has decreased to about 190. In other words, it will take 437,000 aircraft movements instead of the 278,000 originally estimated in the NAMP to serve 87 million passenger trips.
"In addition, from 1997 to 2010 the percentage of wide-bodied freighters decreased from 84% to 67% in favour of medium-sized aircraft. Therefore, moving 8.9 million tonnes of cargo will take 108,000 aircraft movements instead of the 66,000 forecast by the NAMP."
Quite a difference, isn't it? We need 57 per cent more passenger aircraft movements in order to get the same number of passenger trips, and 64 per cent more cargo aircraft movements to move the same amount of cargo.
There is an old transport planners' adage at work here - traffic expands to fill the space available to it. Our experience at Chek Lap Kok has proved it. Airlines are like stray cats. Feed a few hungry ones and you don't get a few well-fed ones. You just get more hungry ones.
Now don't get me wrong about this. If cargo operators take the view that having more space available at Chek Lap Kok allows them to bring in small feeder aircraft, that might otherwise use Guangdong or Shenzhen airports, fine, more power to them. Let commercial sense rule.
But for the life of me, I cannot understand why they should then expect the Hong Kong public purse to pay for their private benefit. That's commercial sense, too, on our part as taxpayers. If the airlines want it, let them pay for it, particularly when it's more a matter on their part of convenience than of real need.
I propose a test of just how much commercial sense it really makes for the airlines to use smaller aircraft. Let's tell the Airport Authority to raise all the money it needs for a third runway off its own resources alone and make the airlines pay for it through higher landing fees.
It's my guess that the airlines would very quickly find it convenient to revert to bigger aircraft and the congestion problem would be much less imminent than it now appears.
Meanwhile, my thanks to Mr Poole for putting down in black and white what was meant for whispers behind closed doors.