[keithro]

When an engine's TBO is 2000 hours.
Is it 2000 hours engine running time ?
Is it 2000 hours bloc time?
Is it 2000 hours airborne?

Does it depend on the maintenance programmed?

[russp]

When an engine's TBO is 2000 hours.
Is it 2000 hours engine running time ?
Is it 2000 hours bloc time?
Is it 2000 hours airborne?

Does it depend on the maintenance programmed?

Differs between engine manufacturers .. see the manufacturers definition.

[johnm]

It is also possible in some circumstances to continue beyonf TBO "on condition" again check with engineers...

[GrahamB]

Speaking for myself, with an O320 Lycoming, it's airborne time, which for convenience is nearly always calculated as 'blocks-off to blocks-on' less ten minutes, and is what use for the aircraft log as well.

For simple non-commercial operations the 2000 hours is not a hard limit; engines can be run on extension albeit with some additional maintenance oversight almost indefinitely.