Lightning breakdown voltage is a significant parameter in judging whether lightning spark is enabled to generate in the fuel tank when an aircraft is struck by lightning. To investigate the lightning breakdown voltage of small gaps in aircraft fuel pipes installation, a test setup is built, and an adjustable impulse voltage generator is adopted to generate the lightning voltage waveform in accordance to the standard SAE ARP5412B. The test results show that the minimum value of critical withstood breakdown voltage measured in the laboratory environment is 9.0 kV at the installation gap of 3 mm. The types of components around the fuel pipes in fuel tank and the installation forms of the fuel pipes are almost not affected by the lightning breakdown voltage of the installation gap. The effects of the lightning breakdown voltage on the fuel pipes in diameter and on the surface of fuel pipes paint layer are negligible. The installation gap of 3 mm should be in good keep. The withstood breakdown voltage value increases from 9.9 kV to 22.5 kV by applying 1 mm thick sealant on the component surface, showing that the voltage amplitude required for gap lightning breakdown is greatly increased. The researches indicate that coating sealant on the surface of fuel pipes is an effective measure to reduce the risk of lightning breakdown in small gaps. The tests results and research conclusions of this paper are available for reference and basis.