If you're flying out of OR Tambo International this South African summer, there's a reasonable chance your flight runs late or your suitcase doesn't travel with you — and it's all down to physics, not airline chaos.

The Citizen reports that SAA, Airlink and LIFT have all flagged that extreme heat can force carriers to cut weight before take-off. Hot air is thinner, so wings generate less lift and engines less thrust, meaning longer take-off runs and slower climbs. At OR Tambo, roughly 1,694 metres above sea level, that combination of altitude and heat is especially awkward.

The numbers are striking. SAA's Mphilo Dlamini says an Airbus A330's permitted take-off weight can fall by up to 20 tons when temperatures climb from 25°C to 40°C. An A320's limit drops from about 77 tons to 73.5 tons over the same range, and an A340's from 261 to 227 tons. Long-haul routes are hit hardest because heavy fuel loads eat into what's left for passengers, bags and cargo — an A340 with 80 tons of fuel might have 44 tons of payload available in cool conditions, but only 10.5 tons at 40°C.

So should you expect to be bumped? Probably not. Airlines work down a hierarchy: cargo comes off first, then fuel is trimmed where regulations allow, then baggage is restricted, and only as a last resort are passengers offloaded. Lifesaving medication and transplant organs are never offloaded. LIFT's chief pilot Warren Penney notes there's no magic temperature that triggers restrictions — each departure is calculated individually based on temperature, air pressure, wind, runway conditions and the aircraft's weight.

Airlines can also shift schedules to dodge the hottest hours, favouring early morning and evening departures, though reshuffles ripple through connections and crew rosters.

The takeaway for travellers: none of this means the plane is unsafe — it's the opposite, a calculation to keep every departure within operating limits. But if you're flying from Johannesburg or another high-altitude airport during a heatwave, expect possible delays, read airline emails carefully for baggage limits, and leave generous buffers between connecting flights. Booking the first flight of the day is a smart hedge.