"..."...Weight management of the F-35B aircraft is critical to meeting the Key Performance Parameters (KPPs) in the Operational Requirements Document (ORD), including the Vertical Landing Bring-Back (VLBB) requirement, which will be evaluated during IOT&E. This KPP requires the F-35B to be able to fly an operationally representative profile and recover to the ship with the necessary fuel and balance of unexpended weapons (two 1,000-pound bombs and two AIM-120 missiles) to safely conduct a vertical landing.
▪▪ The program completed the final weight assessment of the F-35B air vehicle for contract specification compliance in May 2015 with the weighing of BF-44, a Lot 7 production aircraft. Actual empty aircraft weight was 32,442 pounds, only 135 pounds below the planned not-to-exceed weight of 32,577 pounds and 307 pounds (less than 1 percent) below the objective VLBB not‑to‑exceed weight of 32,749 pounds.
▪▪ The actual weights of production aircraft through Lot 8 have increased slightly, with the latest Lot 8 aircraft weighing approximately 30 pounds heavier than BF-44. Weight estimates for Lot 10 aircraft and later project weight growth of an additional 90 pounds, primarily due to additional EW equipment.
▪▪ Known modifications to the 14 Lot 2 through 4 F-35B aircraft, required to bring those aircraft to the Block 3F configuration, are expected to potentially add an additional 350 pounds, which will push their weight above the objective not-to-exceed weight to meet the VLBB KPP. This KPP will be evaluated during IOT&E with an F-35B OT aircraft.
▪▪ Estimates for FoM weight growth include an additional 250 pounds, which will exceed the vertical landing structural limit not-to-exceed weight of 33,029 pounds for the Lot 2 through Lot 4 aircraft. This additional weight may prevent these aircraft from being upgraded to the Block 4 configuration...." page 63
http://www.dote.osd.mil/pub/reports/...2016f35jsf.pdf