"The state of our flying systems remains unsatisfactory," Bundeswehr (German army) chief of staff General Volker Wieker wrote in the report, although he added that the army had managed to "stabilize" its deteriorating readiness.
Germany's Tornados, fighter-bombers designed to fight Soviet forces during the Cold War, are all between 23 and 34 years old and are seen as on their way out of service.
But Defence Minister Ursula von der Leyen insisted that Germany would be able to fulfil its commitments to ally France for the fight against Isis.
"Thirty Tornados are ready for action, and we need six of them. That gives us a wide margin," von der Leyen told broadcaster ARD on Wednesday morning.
MPs in the Defence Committee of the Bundestag (German parliament) are set to debate the report on the state of Germany's main weapons systems later in the day.
Equipment troubles have plagued the Bundeswehr for years, especially the Luftwaffe (air force), several of whose aircraft ran into technical problems while delivering weapons to Kurds fighting Isis in northern Iraq and field hospitals to West African countries battling the Ebola outbreak.
The full parliament is set to vote on Friday on whether to go ahead with the government's plans to join the battle against Isis in Syria with 1,200 soldiers.
As well as the Tornados outfitted with reconnaissance equipment, Chancellor Angela Merkel wants to send an air refuelling tanker and a navy frigate to join the coalition.
Meanwhile, British MPs will vote on Wednesday on whether the Royal Air Force should join French and American aircraft in bombing Isis.