Thrust bump ?
Join Date: Jan 1999
Location: hongkong
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On early V2500-1s, which were a little short of puff especially at high OATs, IAE devised a method to increase the thrust by pushing more fuel into the engine once the take-off roll had started. This was called 'Thrust Bump' and was selected by pressing either of the two guarded pushbuttons on the back of the thrust levers.
The principle is that as the a/c moved forward the ram air reduces the depression in the inlet thereby increasing mass airflow. As this occurs, with a set EPR, the EGT would reduce. Therefore at this point the FADEC automatically introduces more fuel to give higher thrust and 'recover' the lost EGT. Hence more thrust. However it is obviously important to check that this occurs before calling that the trust is correct. Failure to get the TB would invalidate your TO perf figures.
On newer V2500s this TB occurs automatically without selection and if you watch very carefully the EPR demand and the actual EPR rise once the a/c is rolling.
Unfortunately if TB was selected on one engine before t'other was started, then only the selected engine got TB! If the TB pushbutton was pressed on the second engine, once started, you deselected the TB on the first ! Thereafter pressing either button would 'flip-flop' the TB selection between the engines. This could only be fixed by shutting down the one with TB selected and starting it up again. Haven't flown a TB a/c for many years so this may have now been fixed !
The principle is that as the a/c moved forward the ram air reduces the depression in the inlet thereby increasing mass airflow. As this occurs, with a set EPR, the EGT would reduce. Therefore at this point the FADEC automatically introduces more fuel to give higher thrust and 'recover' the lost EGT. Hence more thrust. However it is obviously important to check that this occurs before calling that the trust is correct. Failure to get the TB would invalidate your TO perf figures.
On newer V2500s this TB occurs automatically without selection and if you watch very carefully the EPR demand and the actual EPR rise once the a/c is rolling.
Unfortunately if TB was selected on one engine before t'other was started, then only the selected engine got TB! If the TB pushbutton was pressed on the second engine, once started, you deselected the TB on the first ! Thereafter pressing either button would 'flip-flop' the TB selection between the engines. This could only be fixed by shutting down the one with TB selected and starting it up again. Haven't flown a TB a/c for many years so this may have now been fixed !
