Question

why fountain pen leaks at high altitude? Give reasons.

Answer 1

Basically the leakage occurs due to the change in the air pressure difference, as it less when we are at a higher altitude.So this leads to quick flow of the ink ,appearing like a leakage.
Its due to viscosity ( thickness of liquid). Viscosity is resistance to flow. 

If the viscosity becomes less of any fluid like engine oil & ink, it will flow more easily. 
Higher the temperature of the liquid, lower the viscosity of that liquid. 

Pressure is directly proportional to Temperature, hence as the pressure increases in plane, temperature too increases. And as the temperature increases, it affects the viscosity of ink. Hence, it leaks.