Heathrow’s third runway now comes in two competing versions. Heathrow Airport Limited (HAL) wants a 3,500-metre runway that crosses the M25 to the west, burying a stretch of Britain’s busiest motorway in a tunnel. The Arora Group counters with a 2,400-metre runway that stops east of the M25 and leaves the motorway untouched. The government has chosen HAL’s scheme as the basis for planning, though Arora can still pursue its own application. The real contest is not which runway is longer, but the trade-offs hidden behind the length: cost, capacity, community impact and time.
The case for the shorter runway rests on several arguments. The first is phasing. The long runway crossing the M25 is a single, irreversible undertaking; the runway and airfield alone cost £21bn, and relocating an energy-from-waste plant, diverting rivers and clearing villages to the west must all be paid for up front. The shorter runway avoids the motorway, so it can be built in part now and extended later, in use, if demand actually arrives. The second is cost, estimated at around £23bn to £25bn. The third is capacity efficiency: a short runway can take most narrow-body short-haul traffic, freeing the two existing runways for long-haul, and still delivers more than 80 per cent of the movements the full runway would add. The fourth is the changing fleet. The main purpose of the full 3,500 metres is to let heavy four-engine aircraft such as the 747 and A380 take off at full load, and those aircraft are retiring; the twin-engine jets replacing them on long-haul, such as the 787 and A350, need markedly less runway, which raises the question of whether a third full-length runway is needed at all. Finally, the airlines that pay the charges, including IAG and Virgin Atlantic, tend to back the cheaper option, since the cost ultimately flows into fares.
The arguments against the shorter runway are equally concrete. The first concerns terminals. A runway only adds movements; the passengers on them still need somewhere to check in, clear immigration and collect baggage. Arora’s new Terminal 6 is sized at about 40 million passengers, and its estimate excludes redeveloping the existing central area, so if the shorter scheme later carries passenger numbers comparable to HAL’s, it must still add that processing capacity. The second is resilience. A full-length runway lets any aircraft use any runway, preserving flexibility when a runway closes for maintenance or operations are disrupted; the A380 still flies to Heathrow in numbers, the new 777X is also heavy, and building short locks in a constraint that is hard to reverse for decades. The third is community impact. To avoid the M25, the shorter runway shifts east as a whole, its eastern threshold moving close to a kilometre further east, from west of the village of Sipson to east of it; the government’s assessment found Arora’s scheme uses less land but demolishes more homes, which HAL puts at roughly twice its own figure. The fourth is maturity: in selecting the long runway, the government cited lingering doubts over the shorter scheme’s surface access and engineering readiness.
Several questions remain genuinely open. On noise, Arora claims its scheme is quieter but has published no noise contours; formal modelling will not appear until its Development Consent Order is submitted, expected in 2027, and whether an eastward-shifted runway squares with a “quieter” claim is itself unproven. On construction and planning, the shorter runway is simpler to build and may go up faster, yet the government has placed the long runway on the fast track of the Airports National Policy Statement, while Arora must travel its own DCO route, so planning consent could in fact take longer. On cost, the headline £49bn against £25bn is not like-for-like: HAL’s figure includes £15bn to modernise existing terminals, and whether that spending is unavoidable depends on demand. If traffic climbs towards 150 million passengers, the existing terminals must be rebuilt and the cost cannot be dodged; if demand peaks lower, around 120 million, Arora’s new terminal added to today’s capacity may suffice and that £15bn need never be spent. The two schemes rest on different bets about how far demand will grow.
In sum, the shorter runway’s most defensible advantage, and the one that does not depend on unproven figures, is its phasing and the optionality it preserves, reinforced by the fleet’s drift towards shorter runway requirements. Its claims to be cheaper, faster and quieter vary in credibility and all await formal environmental and independent assessment. What the dispute exposes is a structural truth: a runway is only half an airport. Any comparison that counts the runway but not the terminals it depends on will not show the full picture of which scheme is longer, shorter, dearer or cheaper.
