BAD FORWARD HANGER BEARING
CDB activity was carried out on the AH 64 Apache Helicopter. Initial testing revealed an imbalance on the tail rotor at a level of 0.236 ips. The tail rotor vibration levels were reduced to 0.037 by the CDB team. Further analysis of the data collected during this initial test revealed that a higher level of vibration had been detected at the forward hanger bearing. Several attempts at coarse adjustments utilizing the Number 3 and 4 shafts to reduce this condition had little effect. The CDB computer recommended that forward hanger bearing be replaced. The bearing was replaced and positively identified by maintenance staff as unserviceable on removal.


HIGH VIBRATIONS
At the request of the client, The CDB team investigated unresolved vibration related issues of an Apache aircraft. During the process the team discovered that the APU was loose on its mounts. The APU condition was assessed and it was decided to continue with the CDB assessment as the APU is off when any of the vibration signatures are collected. The aircraft ground runs were carried out with the cooperation of the maintenance Test Pilot and quickly revealed high levels of modulation in the Intermediate Gearbox (IGB) at approximately 103%. Additionally, the readings at the forward hangar bearing gave a high vibration level of 0.567 Ips.
Using CDB technology, a successful series of coarse and fine adjustments were made to Nos 4 & 5 driveshafts (indexing both shafts and adding 7 grams of weight tapes to No 5 shaft) which culminated in an overall reduction of modulation in the IGB to 41% and a reduced vibration level at the forward hangar bearing of 0.209 Ips.
Following the successful process of adjustments the APU issue was addressed.


HIGH VIBRATIONS
After maintenance phase and tail rotor gear box replacement, CDB activity was carried out on Apache aircraft During the data collection process the CDB team detected a high vibrations in the Intermediary gearbox and AFT hanger bearings. Re-indexing has been made to drive shaft no 3 due to additional vibrations in the main transmission gearbox. Modulation (re-indexing and weight changes) adjustments have been made to Intermediate gearbox. Additionally, driveshaft no 6 have been balanced as well as driveshaft no 5. The CDB team recommended checking the lateral vibration in main rotor head on the ground.



HIGH VIBRATIONS
The team of engineers from Shake’d Technologies revealed a slight increase in vibration from previous CDB activity which was carried approximately 400 flight hrs beforehand. Using CDB technology, a successful series of coarse and fine adjustments were made to No’s 3, 4 & 5 drive shafts (indexing shafts and adding weight) which contributed to the overall reduction of vibration levels at main drive train components. Following the successful process of adjustments the CDB equipment was removed and the aircraft is released for 125 flight hrs before CDB re-check is required.




INOPERATIVE ACCELEROMETER & HIGH VIBRATION
During initial CDB run on Apache aircraft, it was discovered that the tail rotor gearbox had a resonant vibration of 1.63 IPS. This vibration level should have illuminated the GBX Vibration message on the UFD. No message was displayed. After CDB process of troubleshooting, the team found that the accelerometer was inoperative in not providing a signal to cause the C/W/A message to appear on the UFD. Additionally, the Intermediary gearbox had a vibration of 1.34 IPS and modulation of 85%. This data should have caused the GBX Vibration message to be displayed as well. The Number 7 drive shaft had a vibration of 0.734 IPS at the accessory gearbox input. CDB vibration limits reveals that any vibration above 0.5 can cause a quick acceleration to an excessive vibration and wear to the over running clutches in the accessory gearbox. Rotation of the input flange by the CDB team brought the vibration down to 0.34 IPS. (see graph) It is important to note that the combination of the TGB and IGB vibrations is an extremely dangerous combination that creates resonant frequency vibrations to travel between the gearboxes along drive shaft the # 6 as the frequencies attempt to harmonize. This harmonic resonance is critical and could result in failure of the drive shaft, TGB, or IGB at some point in the future of this aircraft. After balancing the drive train the vibration levels were reduced to 0.88 at TGB and to 0.553 at IGB as well as reduction to 14% in modulation levels.


HIGH VIBRATIONS AT FWD/AFT HANGER BEARINGS
CDB activity was carried out on the AH 64 Apache Helicopter. During the process a higher than normal vibration levels were detected at the forward and aft hangar bearings along with the input drive of the intermediate gearbox. It was also found that the tail rotor was out of balance.
In order to compensate for the vibration levels detected at the forward hangar bearing, fine tuning weights were applied to the No 3 driveshaft. To compensate for the levels of vibration detected at the aft hangar bearing and the intermediate gearbox input drive, fine-tuning weight tapes were applied to the No 5 driveshaft. The tail rotor was quickly rebalanced the team. The levels of vibration-recorded post the application of fine-tuning weights and the tail rotor rebalance were significantly reduced. A recommendation was made by the CDB team to check the onboard vibration monitoring system for serviceability.


HIGH VIBRATIONS AT AFT HANGER BEARING
CDB activity was carried out on the AH-64 Apache Helicopter. It was quickly established during the process that higher than normal vibration levels were detected at the aft hangar bearing along with the input drive of the intermediate gearbox. In order to compensate for the vibration levels detected at the aft hangar bearing and the intermediate gearbox input, drive fine tuning weights were applied to the No 5 driveshaft. Additionally, a calculated coarse adjustment (Rotation of the shaft) and weight tape were added to the No 4 driveshaft. This action reduced the vibration levels significantly at the aft hangar bearing.
The CDB team recommended that the aft hangar bearing be replaced due to the vibration levels that have been applied to this component. A further recommendation was made that onboard vibration monitoring system be checked for serviceability.




RESULTS