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Combustor Rumble

Combustor Rumble can be a problem in engines that reverse the combustion gas flow prior to

entering 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 ration 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 down the band 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 was negated by the growth rearward as the

engine speed/temperature increased. 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-L-13A 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 0.070 inch.

However a gap of 0.030 to 0.050 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 0.070 inch at assembly. Greater than 0.070 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.

Upon controlling the height tolerance, ATE no longer had any engines with combustor rumble

UNTIL the Deflector with the thermal barrier coating was introduced.

An engine with a TBC coated deflector exhibited a combustor rumble at idle when the gap between the flanges was within the tolerance used by ATE. Assuming the deflector with TBC coating is now 300 degrees F cooler than a bare deflector, it is conceivable that the outer flange of the deflector now grows 0.009 inch less than before. Since no real data exists from Honeywell, ATE reduced the allowable clearance by 0.010 inch and the combustor rumble has been eliminated.

Lately ATE has been receiving engines in for repair or overhaul with engine damage indicative

of combustor rumble. Combustor rumble causes excessive engine vibration that is generally

treated as a transient vibration and acceptable. This Rumble induced vibratory energy is

believed to be the primary root cause for numerous combustor turbine components cracking;

causing significant increases in operator repair costs. Combustor Rumble may be a prime

contributor and cause for:

1. Cracking of exhaust diffuser struts

2. Heavy wear of the aft power shaft spline

3. Loss of the heat shield on the support cover

4. Heavy wear of the support cover where the OD mates with the exhaust diffuser

5. Power turbine bearing wear

6. Heavy wear on the outer flange of the deflector and contact surface with the air diffuser

7. Cracking of the deflector

8. Wear of the pilots on the power shaft and in the power turbine shaft section

9. External brackets breaking.

10. Air diffuser cracking (?)

Combustor rumble was quite evident during the initial development of the T53 with the

atomizing combustor. Fortunately the designer of the Lycoming atomizing combustors had prior

experience with Westinghouse and Pratt and Whitney and Rumble was eliminated in the

T53 by controlling the gap between the combustor outer flange and the deflector outer flange

and raising the Ground Idle speed from 40% - 44% to 48% -52% and that resolved the issue.

The Combustor Rumble number of incidents appear to be increasing, are not thoroughly

understood by T53 operators, or Honeywell; and certainly not recognized by those paying the

extensive cost of replacing the parts most affected by the rumble vibration.

The deflector with the TBC coating is an excellent improvement but the gap between the

deflector and combustor liner outer flange must be controlled. A shim between the position #2 bearing housing and the deflector should be considered and approved by Honeywell.

The dimension/tolerance between the deflector mounting flange and the deflector outer flange must be better controlled.

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