My quota for the week.
As naval aviators we always are prepared for in-flight malfunctions. Considering I was on my fifth flight as an aircraft commander and had experienced a few, I figured I had met my quota for the week.
The weather in the AOR had been less than desirable for the past few months, and we didn't expect it to get better that night. We had received periodic weather updates during the night, but none of them sounded good. When we finished the mission and headed back, the field metro service reported variable winds with blowing dust, visibility between 1,000 and 2,500 meters (tower was calling 500 meters), and a ceiling of 300 feet broken. Though this report didn't sound encouraging, we had become accustomed during a seven-month deployment never to trust the weather at our forward-deployed location.
A VFR combat arrival that evening would not be an option because of weather. We lowered our landing gear before the initial descent and commenced our approach. As we rolled onto final for the TACAN approach at 12 miles, 290 knots at 6,000 feet AGL, the flight station heard a thump, and the aircraft shook violently. I immediately shallowed our descent, leveled the aircraft at 2,500 feet AGL, and slowed to 200 knots. The vibration decreased slightly, coinciding with a decrease in airspeed. Quickly troubleshooting, I advanced the power levers from 1,500 SHP and noticed the vibration intensified. As I brought back the power levers to flight idle, the vibration did not decrease noticeably. We determined the vibrations were not coming through the flight controls, power levers, or the engine emergency-shutdown handles.
We initially thought something had fallen from the aircraft but couldn't determine what it might have been. Our thoughts first turned to the landing gear, because they had been lowered just before the vibration started. The landing-gear-position indicator showed the gear were down and locked, and the aft observer reported the gear appeared to be down. The only other item extended on the aircraft around the time of the initial vibration was the AIMS camera turret, so I had the radar operator retract the turret. This action did not affect the intensity of the vibration, and the turret was reextended. The off-duty flight engineer and pilot then inspected the aircraft cabin, looking through the observer windows to determine if any other aircraft part was missing. They reported that everything looked normal, but the vibration in the back of the aircraft was more severe--to the point it was difficult to stand.
As we neared the airfield, I slowed the aircraft to prepare for landing. We briefly considered not lowering the flaps but decided the slower airspeed was more desirable. When the flaps were lowered, we felt no change in controllability or the vibration. It felt like I was flying a washing machine with a load out of balance. I began to question if we had an airframe issue and reconsidered if something might be wrong with one of the engines.
I decided not to attempt a landing while evaluating the aircraft's airworthiness, so we passed over the approach end of the runway at 2,500 AGL and called missed approach. The copilot declared an emergency and requested tower to make a visual inspection of the aircraft, but they couldn't because of poor visibility. As we turned outbound for our next approach, we discussed whether to postpone landing until we had diagnosed the problem or to land as soon as possible. With the aircraft at a low altitude, in a threat environment, and with deteriorating weather, I decided to land as soon as possible. Our nearest divert field was 120 miles away, and I didn't want to run the risk of making a bad situation worse. At the same time, the TACCO was relaying our situation and courses of action to maintenance control. Because of their proximity to the prop arc, I moved the TACCO and Nav/Comm from their stations, in case the vibration was engine-related.
We reached our approach minimums but couldn't identify the airfield. I executed a missed approach and set 2,500 SHP, and then, very cautiously, set 3,500 SHP on all four engines. On the next approach, the visibility improved, and I could identify the rabbit lights in plenty of time to land, using land flaps. Our touchdown speed was 127 knots, with no discernable change in the vibration.
On landing rollout, I eased back the power levers into reverse and decelerated, still noting no change in the vibration. I stopped the aircraft about two-thirds down the runway. The flight engineer asked to secure engines No. 1 and 2, because it felt like the vibrations were coming from the port side. I agreed, and he secured the engines. Engine No. 1 was shut down, followed by No. 2 about five seconds later. As soon as No. 2 was secured, the vibration immediately ceased. We radioed tower and requested to taxi back to our ramp, about 200 yards away.
As we pulled into our parking spot, a small crowd of maintenance and emergency personnel waited to assist us. When I came down the boarding ladder, everyone had congregated around the No. 2 engine. One of the blades on the engine had lost its cuff (a piece of rubber and honeycombed aluminum that is part of the de-icing system). When the cuff separated, it hit the cowling door on the No. 1 engine nacelle and left a black mark on the backside of a blade on the No. 1 prop. It almost had caused the No. 1 engine to FOD out. Had it separated in the other direction, it would have hit right behind the TACCO station, possibly causing an explosive decompression.
Losing a blade cuff is an uncommon occurrence and not explicitly covered in NATOPS. We were fortunate to have encountered this situation close to the airfield, instead of during a long transit. Good crew coordination and constant communication with ground personnel helped us to recover in a timely manner, without accepting any unnecessary risk.
Lt. Corrigan flies with VP-47.