It would be interesting to know to what extent that is a viable option with HS2 and Heathrow?
@H F Stephens states in a previous post that Heathrow is not running at capacity, but has capacity issues at certain peak times. Are those peak's impacted by a number of regional flights, which could have been replaced by HS2? (I think I read recently that there are 150 flights a week between Edinburgh and Heathrow for example)
Depends on how you define capacity, taken from CAA figures, Heathrow's passenger numbers are down for last few months
April 2026 : 6,721,740 lower than April 2025 : 7,099,310
May 2026 : 7,121,542 lower than May 2025 : 7,204,362
June 2026 : 7,227,536 lower than June 2025 : 7,359,020
July 2026 : 7,859,325 lower than July 2025 : 7,981,535
August 2026 : 7,866,719 lower than August 2025 : 8,036,159
I would guess 8m passengers per month is close to limit, especially as aircraft movements are very limited for around 7 or 8 hours per day (from about 10pm to 6am), with surge of arrivals 6am-8am
Aircraft movements (per CAA table 3) was
April 38,638 (1288 average per day)
May 40,590 (1309 per day)
June 39,415 (1314 per day)
July 41,206 (1329 per day)
August 41,569 (1341 per day)
A few of these will be positioning flights, freight aircraft etc, but around 1350 is probably the practical daily limit (less in bad weather). But clearly capacity is also a function of aircraft size. And unlike 15 years ago, now very few planes with less than about 170 seats use Heathrow.
Apparently mixing narrow body aircraft and wide bodies on same runway is quite inefficient (there are rules about how long a smaller plane must wait before following the disturbed air of a large plane), so that is why Heathrow tends to have 3 holds at start of runway, with planes grouped by size and take off in size batches (smaller planes join runway and start few hundred feet ahead of the heavies)
If a third runway was opened, (even a shorter one of about 2500m), could move the narrow body planes to it, and use the others more consistently, although Heathrow seems to be aiming for 3 full length runways that allow mixed mode on all.