The plane has landed safely in Cologne, the spokesman told CNN on Friday.
Air force Col. Guido Henrich said the problem was caused by the failure of an electrical distributor box, which affected the radio system and also a fuel system, meaning that the plane couldn't dump fuel. The delay forced Merkel to miss the start of the G20 summit and a scheduled meeting with President Donald Trump.
The report said that the fire brigades were dispatched in the event of a hard landing.
The Airbus A340 government aircraft is named Konrad Adenauer after West Germany's post-war chancellor.More news: Trump reaffirms plan for second North Korea summit plan
He stressed, however, that there was no danger to the people on board.
The German air force ruled out sabotage, with a spokesperson saying "there is absolutely no indication of a criminal act".
A senior journalist with the public national television ZDF said Merkel would spend Thursday night in Bonn. Threat to the passengers on Board the plane "Konrad Adenauer" was not, she assured. Later, Merkel would say she thought something bad had happened in Germany. News portal Spiegel Online reported the plane suffered a complete failure of its communication system that left the crew without contact with ground control. In 2015, the same aircraft was grounded due to a technical problem shortly before Merkel was to fly to New Delhi, forcing her to switch to a distinctly less luxurious A310 troop carrier.
In a freakish incident involving Merkel's official plane in 2013, a 24-year-old intruder high on drugs and wearing just underpants managed to get aboard the parked jet in Cologne and stage what media termed "a one-man party" before he was arrested.