Boeing 747-400 Engine view in heavy rain

A Virgin Atlantic Boeing 747-400, G-VROM, takes off in heavy rain — spray streaming off the wing and the engines running at takeoff thrust through a downpour.

Rain does not bother a jet engine

That is the counter-intuitive part. A high-bypass turbofan the size of a 747’s ingests an enormous mass of air every second, and the water in even torrential rain is a tiny fraction of it by weight. Most of what enters is flung outward by the fan and passes down the bypass duct rather than into the core.

Engines are certified for it deliberately. Manufacturers run water ingestion tests, firing measured quantities of water into a running engine to prove it will keep operating — part of the same certification campaign that includes the extreme testing regime the airframe goes through.

Hail and volcanic ash are a different matter entirely. Water is not.

What rain does affect

The runway. This is the real operational concern, and it is about stopping, not about the engines. Standing water reduces braking friction and raises the risk of aquaplaning, which is why performance calculations use different figures for wet and contaminated runways, and why crews may need a longer runway or a lower takeoff weight in rain.

It is also why a rejected takeoff on a wet runway is one of the most demanding things a crew can be asked to do.

Visibility. Rain heavy enough to be seen like this generally comes with reduced visibility, and often with the convective weather that produces windshear.

What you can see on the wing

The 747-400’s wing carries six-foot wingtip extensions and six-foot winglets, the feature that most quickly separates a -400 from the classic 747s. In rain the airflow over the wing becomes briefly visible — water streaming aft along the surface traces the flow the aerofoil is generating.

Related

• The 747-400
• Reverse thrust on a wet runway
• What a 747 engine does to a car