Vice Adm. Carl Chebi, the head of Naval Air Systems Command (NAVAIR), said on Apr. 4 that the F-35C has seen a “significant improvement” in mission capable and availability rates over the last year.

The C is the carrier variant of the Lockheed Martin [LMT] F-35 fighter.

“Our target number of aircraft is 38. This morning, we had 36, ” he said in a brief interview after his participation in a NAVAIR forum at the U.S. Navy League’s Sea-Air-Space conference in National Harbor, Md.

Lockheed Martin builds the F-35’s forward fuselage and wings and assembles the plane at its Fort Worth, Texas plant. In addition, the company is responsible for “sustainment support,” elements of which include the supply chain, the logistics system, depot maintenance and pilot and aircraft maintenance training.

Chebi outlined the four “primary cost drivers” for sustainment, which he said represents 75 to 80 percent of the cost of the F-35 program–“sustainment support” by Lockheed Martin and Raytheon Technologies‘ [RTX] Pratt & Whitney, which builds the F135 engine for the fighters; organic maintenance; spare parts; and engine parts.

“Sustainment support is the number of Lockheed Martin and Pratt & Whitney folks we have,” Chebi said. “We’ve reduced a lot of those through training.”

Reducing sustainment costs for the F-35 while improving the availability of the fighter for missions is a perennial issue.

In November 2019, then DoD acquisition chief Ellen Lord told the House Armed Services Committee’s readiness panel that driving down F-35 support costs “is the key lever in reducing overall F-35 sustainment costs because sustainment support accounts for over a third of all sustainment costs.”

report by the Congressional Budget Office (CBO) in February said that between 2021 and 2022 F-35As’ availability fell by 11 percentage points, while F-35Bs’ availability fell by 7 percentage points, and F-35Cs’ availability rose by 5 percentage points.

Last year, availability rates ranged from 54 percent for the F-35A and F-35B to 58 percent for the F-35C, CBO said. Availability rates are calculated by dividing the number of hours that aircraft are both mission capable and in the possession of operational squadrons by the total number of aircraft hours for the entire fleet, including aircraft undergoing depot-level maintenance.

Repairing stealthy aircraft, such as the F-35 and the Lockheed Martin F-22, may involve removing and then reapplying stealth coatings–a process that lengthens repair time and decreases aircraft availability rates.

CBO said that the availability rate of an F-35A with five years of age was around 40 percent, while an F-22 of that age had an availability rate of more than 50 percent, and an F-15E of more than 70 percent. In addition, while F-15Es recorded about 25 flying hours per month at five years of age, F-22s and F-35As of the same age recorded about half that number.

The F-35 program is looking to award a five-year Performance-Based Logistics contract to Lockheed Martin this year (Defense Daily, Feb. 17). The PBL contract is to incentivize the meeting of goals for F-35 for Gross Issue Effectiveness (GIE) and Supplier Response Time.

GIE measures the delivery of the correct parts to the right place at the right time.