I used to say there were only two things that worried me while flying an airplane: in-flight fire, and structural failure. Both are unpredictable, can occur with very little or no warning, and are insidiously dangerous. SwissAir 111 was an in-flight fire that quickly consumed the crew all aboard perished. American 587 was a structural failure that occurred after take-off when the vertical stabilizer separated from the aircraft. All aboard perished.
Those are dire and dramatic incidents, but they happen and they can happen again. These aren’t the only two accidents of this nature to occur but sometimes a better outcome is possible. In-flight fires can be fought, maintained, or extinguished. Structural failure doesn’t always lead to a loss of aircraft control. Aloha 243 suffered massive structural failure with a large piece of the fuselage separating from the aircraft and that plane landed safely with one casualty, a flight attendant. She was never found.
Nevertheless, these types of incidents are possible and are always something that I think about and wonder “what would I do?” or “will I even be able to react?” The worst thought for myself is encountering an incident where I don’t even have time to react to manage the situation. As pilots we all hope that we have the wherewithal to react appropriately and successfully manage the situation to a safe landing.
In light of my two fears, I’m not sure if I’m adding a third or just re-classifying my structural failure into a slightly broader category. In the past 18 months I have had two incidents of in-flight vibration that persisted through multiple phases of flight. This is significant because usually vibration is caused by the flight controls or the engines. If a power change doesn’t change the vibration then it’s likely the flight controls as they are the only parts that are supposed to be movable. If it turns out it’s not the flight controls then we have a bigger problem and structural failure could be next.
Last summer I was operating a ferry flight from Chicago to Raleigh-Durham. The aircraft had just come out of maintenance and they had either replaced or re-rigged the ailerons. The ailerons being the “little wing” on the trailing edge of the outside portion of the wing. These are the control surfaces that move in unison to roll the aircraft along the longitudinal axis.
I don’t usually worry too much about an aircraft just coming out of maintenance. I don’t have many superstitions when it comes to flying. I might avoid a 666 squawk code or flying at 13,000 feet if I have to. But maintenance isn’t something I worry about.
We actually had to drive to the hangar to pick up the aircraft which I guess should have been a bigger clue to pay close attention to the aircraft maintenance history. I didn’t actually know why it was at the hangar until after the event happened.
The Captain and I arrived at the aircraft and began our preparation to fly to Raleigh-Durham. Besides the fact that we would have no passengers, this would be a normal flight. The flight attendants were already on-board and ready to go. The plan was to fly to Raleigh-Durham and pick up a load of people that got stuck overnight and fly them to Newark. Standard stuff. Nothing out of the ordinary.
Once we figured out the slightly convoluted maintenance hangar ramp procedures, we were ready to taxi. After we started both engines, we began our long taxi to the departure runway at O’Hare. From the hangar to runway 22L was about the longest taxi you can get for departure. By the time we were ready to depart the brake temperatures were approaching the maximum limit for take-off.
As long as they don’t exceed 300 degrees, we can take-off. However, what happens when you depart and put the gear up into the closed off gear wells? Usually they get warmer. This meant that after we took-off we would leave the gear down for a few moments to allow them some extra time to cool off.
What really happened is out of habit when we took-off we put the gear up. Then the Captain regretted that decision and wanted to put the gear back down. Which required us to slow the aircraft back down below gear extension speed.
Isn’t this about in-flight vibration? Yes. Good focus.
The gear did two things to us in this situation. It distracted us from any abnormal cues, and it required us to slow the aircraft down. For a few minutes during our initial climb our focus was on the gear, the airspeed for gear extension, and the brake temperature. Once we cooled off the landing gear and put the gear up, we accelerated to our normal climb speed.
Which is where life got interesting. Once the airspeed was above 300 knots in the climb, we felt a very consistent and very noticeable vibration. I was actually out of my seat when the vibration first started. Being a ferry flight, I had walked to the galley to pour myself another cup of coffee. It was like a scene from a movie, I am pouring my coffee and the plane started vibrating and my coffee cup shuddered in my hand.
“That’s odd…” I thought to myself as I walked back into the cockpit.
“What the hell was that?” I said to the Captain.
“I don’t know.” He said looking at the instrument panel, head half-cocked to the side like a curious puppy.
Airplanes do a lot of things at different times during flight. Vibrate ain’t one of them. The engines produce very small vibrations that can sometimes be felt in waves as they are out of sync slightly. The low hum of the outside wind flying around the fuselage is constant and loud. Airplanes do not vibrate like a washing machine with a lopsided load. They shouldn’t. Ever.
What you’re really feeling is an oscillation. Sometimes oscillations get worse and worse until something departs the aircraft for good then who knows what will happen. Sitting down in my seat and putting my seatbelt back on I had the feeling of “this is not good.” I put my coffee down and started looking around for clues.
The airbus has very good system synoptic pages that tell us a story about the aircraft and what it’s presently doing or not doing. We can look at the flight controls on our display and see if they’re moving in ways they shouldn’t be (fluttering in the wind, creating the oscillations or vibrations we’re feeling). They weren’t. We can look at the hydraulic systems to see if they’re over-pressurized and we’re feeling some feedback through the fuselage from the hydraulic pumps or lines. We’re not. We can look at the pressurization page to make sure the pressure vessel is intact and we’re not feeling the inside trying to fly to the outside. We’re not.
All this looking and we’re no smarter than the puppy with his head half-cocked.
We slowed the airplane back down below 300 knots and that seemed to alleviate the vibration significantly. Well that’s a step in the right direction. As of right now we have no idea what’s causing it or how to fix it but we can make it less dramatic by slowing down. This is good news and it’s an immediate relief of sorts. We’re still not happy but we’re not concerned about pieces departing the aircraft or losing control of the aircraft.
This is when we start making decisions about where we’re going. We very clearly are not going to Raleigh-Durham anymore. Are we going back to O’Hare? Are we landing immediately? Should we declare an emergency?