The Boeing E-4B "Nightwatch"

The Boeing E-4, project name Nightwatch, is an aircraft built for a single grim purpose: to make sure that if the places from which the United States commands its forces are destroyed, the command itself survives.

Four were built, all on 747-200B airframes, and all four remain in service.

What it is for

Fixed command centres have a fundamental weakness — their coordinates are known. An airborne command post does not, because it can be anywhere within a very large volume of sky and can stay there for a long time.

The E-4B is designed to carry the National Command Authority and its staff, receive intelligence, and issue orders to forces worldwide, entirely independently of ground infrastructure. It is officially the National Airborne Operations Center (NAOC), and in peacetime it also serves as a flying operations centre for the Secretary of Defense on overseas travel.

What is on board

The interior bears no resemblance to an airliner. It is arranged as a working command centre across three decks, with a briefing room, a conference area, work areas for a battle staff of dozens, and a communications section.

The communications fit is the point of the aircraft. It can reach forces across the spectrum from satellite links down to very low frequency radio — the band used to reach submerged submarines — using, among other things, a trailing wire aerial several miles long deployed in flight.

Hardened

The E-4B is built to keep working in conditions that would disable an ordinary aircraft. Its systems are shielded against the electromagnetic pulse produced by a nuclear detonation, which would otherwise destroy unprotected electronics at considerable distance.

It can also be refuelled in flight, so its endurance is limited by crew fatigue and consumables rather than fuel.

Recognising one

From outside, the giveaway is the large dorsal fairing immediately behind the upper deck, which houses satellite communications equipment. Otherwise it looks much like any early 747 — which is, presumably, not a disadvantage.

Still flying

The E-4B fleet is now decades old and based at Offutt Air Force Base in Nebraska. Replacing it has been repeatedly studied, most recently under the Survivable Airborne Operations Center programme, but the four aircraft remain in service — among the oldest 747s still doing serious work anywhere in the world.