The US National Transportation Safety Board has launched an investigation into the late night take-off of a Bombardier Learjet 35A from a closed, unlit runway at Washington Dulles International airport.

The Learjet, owned by Florida-based charter and air ambulance provider, National Jets, was cleared on to Runway 19R at 03:13 by a Dulles tower air traffic controller and successfully departed soon after, with four people on board. Two controllers were assigned to the cab, although one was on break. The NTSB is investigating the incident as an "operational error".

The airport earlier had closed the runway for surveying work and turned the lights off, the NTSB says. The closure was broadcast on the airport's automatic terminal information service, a recording pilots listen to before taxiing. The tower controller had placed an "X" on the tower's ground radar display and had annotated the information on the tower status display as reminders. An approach controller working in a different facility nearby noticed the error on radar as the jet departed.

It is not clear why the pilots accepted the take-off clearance, and National Jets did not respond to a query about the incident. National Jets had recently earned the ARG/US Silver safety rating for its Learjet fleet, according to the company's website.

Source: Flight International