Combustor Rumble

Combustor Rumble can be a problem in engines that reverse the combustion gas flow prior toentering the 1st stage gas producer nozzle. Audible Combustor Rumble is created by

unstable combustion, in addition to creating a significant increase in engine vibration during

combustion instability. Unstable combustion occurs when the Air/Fuel ratio is less than 50 to 1. It is similar to the audible rumble heard when adjusting the air quantity on an oil furnace.

The air quantity is being set by closing the band, or by rotating the plate on the air intake until rumble is induced, then opening to stop the rumble.

In the T53, the gas flow deflector grows axially forward from the inner diameter and axially

rearward at the outer diameter. The growth forward is negated by the growth rearward as the engine speed/temperature increases. In contrast, the combustor liner outer wall and forward

flange grow axially forward as temperature increases, effectively closing the gap being


Lycoming checked this gap dimension during the first few years of production of the T53-L13A and may have deleted it in later production engines. There is no Honeywell requirement to check or maintain an allowable gap.

Assuming the combustor liner outer wall is at 1600 degrees F and the outer wall of the

combustor housing is at 500 degrees F, the forward flange of the combustor liner grows

axially forward 0.052 inch when at max power. The cold build gap is typically at 0.030 to 0.090 inch. However, a gap of 0.050 to 0.080 inch at time of assembly is desirable.

ATE has been using Global to set/reset the Deflector outer flange height to control/produce a maximum allowable gap of 0.080 inch at assembly. Greater than 0.080 inch will result in a

combustor rumble at Ground Idle and sometimes as high as 60% N1 speed. Prior to Global

tightening the allowable tolerances, approximately 20% of the engines serviced would have a combustor rumble requiring the deflector to be changed to one having a lower height

dimension. After controlling the height tolerance, ATE no longer had any engines with a

combustor rumble UNTIL the Deflector with the thermal barrier coating was introduced. An