The US Air Force ordered the grounding of its entire fleet of Rockwell B-1B Lancer bombers on 7 June due to safety concerns.

The USAF suspended flights of the aircraft following an emergency landing of a B-1B at Midland International Air and Space Port in Texas on 1 May, which had blown at least one of its escape hatches without launching its ejection seat. All four members of the crew landed safely.

An air safety investigation board is looking into the mishap and aircraft will return to flight as the issues are resolved, the USAF says.

“The safety of airmen is the command’s top priority,” said the USAF. “The Air Force takes safety incidents seriously and works diligently to identify and correct potential causes.”

The USAF has not disclosed suspected causes of the ejection seat mishap.

The B-1B is a strategic bomber that no longer is equipped to carry nuclear weapons. Instead, the USAF uses the B-1B in undefended airspace and to launch long-range missiles at targets on ground or at sea.