From January through August 7, 2017, F-35 aircraft were unable to fly about 22 percent of the time due to parts shortages stemming from incomplete plans and funding.
Spare parts for Lockheed Martin Corp's stealthy F-35 fighter jets are not keeping up with demand according to a report out on Thursday from the US Government Accountability Office.
The report said "from January through August 7, 2017, F-35 aircraft were unable to fly about 22 percent of the time due to parts shortages."
A Lockheed representative said the company is working with the F-35's programme office to understand every factor of the cost per flying hour and identify disciplined ways it can reduce the overall operations and sustainment costs of the F-35.
The shortages are expected to plague the fighter jet programme for several years the report said.
Partly to blame were "incomplete plans and funding that did not account for the long lead time parts." In other instances, delays in the establishment of repair capabilities were blamed.
The US Department of Defense is maintaining more than 250 of the advanced F-35 jets and plans to triple the fleet by the end of 2021.
In February, the Pentagon made a deal for a batch of 90 of the fighter aircraft agreeing to pay below $95 million per F-35A model jet for the first time, compared with $102 million in the previous purchase.
The F-35 comes in three configurations: the A-model for the US Air Force and US allies; the B-model, which can handle short take-offs and vertical landings for the Marine Corps and British navy; and the carrier-variant F-35C jets.
Ten other nations including Australia, Denmark, Israel, Italy, Japan, the Netherlands, Norway, Turkey, South Korea and Britain are also customers.
Lockheed, the prime contractor, and its partners including Northrop Grumman Corp, United Technologies Corp's Pratt & Whitney and BAE Systems Plc have been working on building a more cost-effective supply chain to fuel the production line in Fort Worth, Texas.
In all, the Pentagon expects to buy 2,457 jets.