This is in the TB21 (turbo IO540) POH – above 15000ft.
The TB20 doesn’t have the requirement.
Boiling point lowers with altitude and you can get fuel pressure fluctuations?
… or even vapour locking?
The membran pump in engine is able to make the same relative pressure to atmosphere pressure/ or exactly to crankcase pressure. So absolute pressure is less and less when climbing higher. Somehow nozzle does not spray anymore like on sealevel altwough the pressure difference of fuel and intake air remains same or because of lower amount of fuel sprayed.
Something to do with manifold pressure (turbo boost) getting higher relative to ambient at higher alt?
Peter wrote:
This is in the TB21 (turbo IO540) POH – above 15000ft.
The TB20 doesn’t have the requirement.
The TB21 has power at this altitude (=needs more fuel), the TB20 doesn’t (sorry, I couldn’t resist 
)
