March 7, 2013

What is that thing on the MD80 nose wheel?

"Hey guys, you appear to be dragging something."

There are a number of odd things about the MD80 that result in frequent questions from passengers and aviation enthusiasts...I've already written about two of my favorites, the one-up one-down elevator and the difficult to find wet compass. Another question I hear on a regular basis is in reference to "that thing" I'm dragging from the nose wheel.


The question usually comes from passengers, since most avgeeks (that's a term of endearment by the way) and tower controllers are already familiar with this unique aspect of the MD80. So on a recent flight when the question came from someone I considered to be "in-the-know," I found myself wondering if I really was dragging something.

I was the First Officer on an MD80 departing Chicago O'hare on my way to Phoenix, Arizona. About the time we reached cruise altitude, Chicago Center relayed a message from a tower controller back at O'hare who wanted us to know that we had apparently been dragging something before we departed. The captain and I were surprised by the this information and couldn't imagine why the controller would not have said something to us before we took off. We were also thrown off because the message indicated that whatever we were dragging was attached to one of the main wheels. If we had been told something was attached to the nose wheel then we would most likely have disregarded the message. We are, after all, used to that one...even from an unlikely source.

We had a long flight ahead of us with plenty of time to decide how we wanted to handle the situation...no need to return to Chicago.  If there was something wrong with the jet, they could certainly handle it in Phoenix.  After a while, we decided what we really wanted was more information, so we got on the phone with our dispatcher and asked for clarification. Which main wheel...left or right?  How big was this thing?  Was it sucked up into the wheel well as the gear came up?



The picture above is the left main wheel of the MD80 I was flying that day.  As you can see, there is a similar, but significantly smaller contraption attached.  The debris deflector is designed to keep lose objects from the runway and tire fragments following a blown tire on takeoff from being ingested into the engine.  Oddly enough, I can't ever remember anyone asking what it is or why it's there.  I suppose it's small enough not to be noticed.

As it turned out, the question had not originated from the Chicago control tower...they had simply relayed information that was somehow sent to them from the terminal. A passenger waiting for his flight saw us taxi past and noticed this "thing" hanging from our nose wheel. That's right...nose wheel. Most of the information we had received until now was incorrect. The whole thing reminded me of a game I played as kid where we all sat in a circle and whispered some phrase from person to person until it got back to the person who started. The message that got back to the original sender never sounded anything like what was started. The message sent our way had definitely been lost in translation.

The "thing" we were dragging was actually a spray deflector designed to minimize water and slush ingestion into our tail mounted engines during takeoff and landing.  There's a long list of airplanes with aft mounted engines that do not require a spray deflector, so the question is...why does the MD80 need one when other similarly designed jets do not? In all honesty, the answer to this question is not one that has ever been explained to me by my employer and I haven't had any luck finding an official answer.

My guess is that the unique geometry of the MD80 makes the spray deflector a necessity. The length and shape of the fuselage, the location and height of the wing, and the distance from the nose wheel tires to the engines must present a unique situation that makes water and slush ingestion possible on this particular airplane.  McDonnell Douglas' answer to the problem was the spray deflector.




Another option to prevent such ingestion is the shape of the nose wheel tires. Pictured below is the nose wheel assembly on a Boeing 727. Take a look at the nose wheel tires and you will see that the tires are designed with a "chin" that serves to deflect fluid away from the 727's aft mounted engines. The chinned tires on the 727 are lighter than the spray deflector on an MD80, but they are also more expensive. The two aircraft makers...Boeing and McDonnell Douglas...had to make their own decisions about where to spend the money. Up front on the spray deflector or down the road on more expensive tires.



While the chinned tires on the 727 may be more expensive, there is one significantly negative aspect of the MD80 spray deflector in that it has been know to cause problems with the normal extension and retraction of the nose landing gear. The following is from a September 2003 NTSB report:

"On September 2, 2003, a McDonnell Douglas DC-9-82 (MD-82) was substantially damaged during an emergency landing at the John F. Kennedy International Airport (JFK) after the flightcrew was unable to extend the nose landing gear.  While on final approach to EWR, the landing gear was lowered; however, the nose gear indicator red light remained illuminated and the nose landing gear indicator pin did not extend. The First Officer, who was flying at the time, executed a missed approach and flew air traffic control vectors while the Captain attempted to troubleshoot the problem. After performing emergency checklists, which included the emergency gear extension checklist, the captain diverted to JFK, and performed a low approach over runway 4L, a 11,351-foot-long, 150-foot-wide, asphalt runway. Ground personnel who observed the airplane stated that the nose gear doors were partially open, however, the nose gear was not visible. The captain subsequently performed an emergency landing to runway 4L, with the nose landing gear retracted. After the airplane came to a stop, the passengers deplaned via the aft door exit.  No injuries were reported from the 2 pilots, 3 flight attendants, or the 133 passengers on board at the time of the incident."






Investigators discovered the polyurethane nose landing gear spray deflector "fractured near the middle, and found in two sections. The right section of the spray deflector had rotated about 180 degrees, and was found wedged between the nose landing gear and the right side of the wheel-well structure. The nose landing gear tire was above the wedged spray deflector section. The left section of the spray deflector was found in it's original position with the nose landing gear retracted." 1

Damage to the spray deflector was determined to be the cause of the incident. You may now consider yourself to be in-the-know. No need for alarm.  We aren't dragging anything that isn't supposed to be there.

3 comments:

  1. I'm a degreed engineer and also an Exec Plat and lifetime Plat on American, earned the hard way with butt-in-seat mileage. In other words, I've flown a lot. What should a knowledgeable passenger bring to the captain's attention? Years ago I had an exit row window seat on an US Air 737-200. When flaps and slats were extended on approach, I noticed that one of the slats on my side did not extend -- and it didn't extend until the end of the rollout, when it simply dropped into place. I mentioned it to the captain when I deplaned, and he seemed very thankful. How would he have known about the bad slat actuator otherwise? Maybe he could feel a slight yaw because of asymmetry. Anyway, the question is, when should a passenger say something and when should a passenger not?

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    1. Excellent question Chuck. I joke about people asking questions I've heard a hundred times, but I don't mean to suggest that you should not share your concerns if you have them. I'll try to write a post about this sometime soon. Thanks for your comment.

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  2. From another layperson, though a recreational pilot, I would think that if you have a reason to think there's a safety issue of which the flight crew isn't aware, you should bring it to the attention of the crew.

    A few years ago, I was seated by the upper-deck exit door on a 747-400. During the flight, I found that the inside of the storage bin beside my seat was extremely cold (on the wall, immediately aft of the door), and I could feel a bit of a draft. Wondering if there might be a problem with the door seal, I spoke to a flight attendant in the galley and told her of my concern. I think it's a reasonable thing to do, and if you don't jump up and down and create a panic, then you've done your part to bring an issue to their attention.

    That said, if a plane on which I'm a passenger is lined up ready for takeoff and the flaps and slats haven't been extended yet (as in NWA flight 255), I will raise 31 flavours of holy hell if I think they've been forgotten. I always look, just in case.

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