 A further engine problem was that of uncommanded opening of the variable thrust nozzle (usually through loss of engine oil pressure, as the nozzles were actuated using engine oil as hydraulic fluid); although the engine would be running normally at high power, the opening of the nozzle resulted in a drastic loss of thrust to below that required to maintain level flight.At low altitudes this was unrecoverable, and it caused the loss of at least seven F-104s during testing.A modification program installed a manual nozzle closure control that reduced the problem, but according to designer Kelly Johnson, this took nine years to resolve satisfactorily.