Ask the pilot

Exploding tires and bad math; the pilot exposes his darkest secrets.


Patrick Smith
October 14, 2005 4:30PM (UTC)

Here's a letter:

"Patrick Smith, your credibility as a commentator on all things aviation-related would be more sustainable if you'd get the physics right. Your reference to a 'forward-acting force against your body when applying the brakes in your car,' being akin to the force that brings down the nose of a landing plane, is complete hogwash.

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"First, there is no forward-acting force when braking in the car. Your inertia keeps you moving forward, but the force you feel is a reverse-acting force exerted by whatever part of the car you're in contact with (safety belt, most likely), to keep that inertia from sending you into the windshield. More importantly, this is only indirectly related to the lowering of an aircraft's nose after touchdown. Two things bring the nose down: the force of gravity pulling the front of the plane toward the earth, and torque rotating the aircraft around the rear wheels, caused by application of the brakes. The reason for the JetBlue pilots not to brake heavily was to allow forward speed to bleed off slowly, thus allowing residual lift on the nose, and torque tending to rotate the nose upwards (generated by air acting on the wings and horizontal stabilizers), to yield to gravity as gradually as possible."

You mean that's not what I said? The above scold (I cleaned and paraphrased it slightly), comes from Steve Demuth of Decorah, Iowa, calling me out for my description of why the pilots of JetBlue 292 -- the too-famous Airbus with the crooked front tires -- were obliged to take it easy on braking and reverse thrust upon landing at LAX. Letting the nose fall gently was, Demuth explains, all about torque and gravity. Everybody clear?

With respect to the inertia thing, indeed the force a driver feels when jamming on the brakes is technically a backwards-acting force, but describing it as such would have thrown readers off. You perceive yourself being hurled forward. One option to "forward-acting force" was to type "forward-feeling force" -- a phrase of such alliterative horror that I went ahead with the lie.

Besides, what do you expect from a math and physics flunky? It's funny, because I often get letters from aspiring pilots-to-be worried that below-average mathematics skills might keep them grounded. There's a lingering assumption that airline pilots are required to demonstrate some sort of Newtonian genius before every takeoff -- a vestige, maybe, from the days when airmen carried slide rules and practiced celestial navigation. Dear Patrick, I'm a high school junior who hopes to become a pilot, but my C-minus in honors-level pre-calculus has me worried. What should I do?

What these people don't realize is that I would have killed for a C-minus in elementary algebra. (My final semester report card from St. John's Prep, class of 1984, reads something like this: B, B, C, D, D, C.) I can only vaguely define what pre-calculus might be, and I frequently struggle to make change for a dollar or add up my Boggle scores without use of a calculator. In spite of this, I never graded lower than a 97 percent on any FAA written exam, and no fewer than four airlines considered me worthy of wearing four stripes (before going out of business or laying me off, but still). My logbook records no math-related incidents or catastrophes.

Not that I couldn't use some tutelage with the basics -- and basics are what pilots encounter. At my last job, for example, routine arrival assignments demanded some quickie mental arithmetic. Modern flight-management systems will hash out descent profiles automatically, but on the classic model 737 we had to run the data in our brains: "OK, if we need to be at 14,000 feet in 60 miles, assuming a 2,000 foot-per-minute descent and 320 knots groundspeed, at what point should we start down?" Like an SAT question, but with hundreds of passengers assuming you're smart enough to know the answer. Not to mention an ornery captain, who'd eye me suspiciously as I reached for the old Texas Instruments. "What, you can't figure that out?"

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"Sure I can. I'm just more comfortable this way."

Flying as the second officer on old cargo jets, my duties included the ciphering of weight-and-balance figures and the en route management of fuel (in addition to the more important duties of serving dinner and emptying the trash). On a flight from the United States to Europe, our total weight approached half a million pounds, with tens of thousands of pounds of kerosene to balance among eight tanks. In other words, there were lots of numbers. They didn't require anything too elaborate -- I'd add them, subtract them, portion them in half or a quarter -- but they were big, six-digit affairs that were constantly changing.

With that in mind, look closely at this photograph (snapped early one morning before takeoff for Brussels, Belgium) of the second-officer workstation of the DC-8-73F and try to pick out the most critical cockpit instrument of all. Hint: It was not furnished by the designers at Douglas, who conceived this hideous ark of a plane back in the mid-1950s, when men were men and could fly and do long division at the same time. I'm referring to my $6.95 calculator from CVS -- the one flight bag accessory more indispensable than an emergency checklist, aircraft de-icing guide or bag of ramen noodles. You can see it on the lower shelf, just to the right of my plastic flashlight, marked by a Day-Glo orange sticker affixed in mortal fear that I might otherwise leave it behind.

And hang on, fumbling over the physics of deceleration wasn't my only mistake:

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"Patrick, your analysis of the JetBlue 292 mishap gives the false impression that landing gear anomalies are by nature innocuous. As you should know, some are and some aren't. Remember, it was a blown tire that led to the Concorde crash some years ago."

That's a good point, actually, though it tends toward apples and oranges; nose gear versus main gear. Malfunctions involving a plane's forward (nose) landing gear are by nature innocuous. Those involving main-gear tires beneath the wings and fuselage? Well, that's a little different, and occasionally dangerous.

How so? Failure of a main-gear tire at high runway speed, particularly on takeoff and when a plane is heavily loaded, can induce all sorts of trouble, from greatly reduced braking capabilities to the chance of explosion or fire. Making things worse is the possibility that a single failed tire will propagate the failure of those around it. On most larger planes, the main-gear units are split into bogies of four tires each. On the 747 for instance, 16 wheels are grouped in four separate assemblies. On the 767, 757, and Airbus A330, among others, you'll find two groups of four. It's not unheard of for a blown tire on one bogie to quickly become a blowout of two, three, or four.

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Ironically, with JetBlue in mind, tire trouble is, to a pilot, considerably scarier on takeoff than on landing -- the velocities are higher, the plane is heavier, most of the runway is behind you, etc. A high-speed runway abort with multiple expired tires can be a dicey operation. For this reason, a crew will in most cases opt to continue the takeoff, if at all possible, and deal with the problem once airborne.

Not that it happens very often, and most modern airliners are protected with highly sophisticated anti-skid systems, brake-temperature readouts in their cockpits, and wheel-well fire suppression systems in the landing-gear bays.

Notice I say "modern." One such plane sorely lacking such features was that old DC-8 freighter I used to fly. Ask the typical flier what he or she fears most, and you'll hear things most pilots rarely worry about: turbulence, lightning, wings snapping off. People don't think much about landing gear, but after almost four years of crewing that ancient Douglas I'd developed a hierarchy of potential nightmares, and tire disasters were tops on the list. A story:

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Late one night in 1998 we were going from Brussels to New York. The plane was at its highest allowable taxi tonnage and the ground controllers gave us a long, circuitous route to runway 25R. Rolling along the apron in predawn darkness, we suddenly heard a bang and felt a shudder. A small pothole, we concluded, and kept going, as otherwise the aircraft felt normal.

Then, just as we turned onto the runway and were cleared for takeoff, we heard a second bang, followed rapidly by a third, and then a fourth. And with that, the airplane -- all 355,000 pounds of it -- seized and wouldn't move.

Turns out the first noise we'd heard was one of the DC-8's eight main tires violently giving up the ghost (like a 757 or 767, they are paired in two sets of four). At max weight and after several sharp turns along the taxiways, it was only a matter of time before the adjacent one met a similar fate. With two gone, stress on the remaining two sent them popping as well. We were lucky things happened when it did, and not at 150 knots, with the threshold lights fast approaching.

The runway was closed for more than seven hours until the crippled plane could be unloaded, de-fueled, and towed away for repairs.

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Aircraft tires are filled with inert nitrogen, not air, to reduce the likelihood of explosion or fire, and fuse plugs are designed to trigger automatic deflation should temperatures exceed a certain value, such as following an abort. In 1986, a Mexicana 727 went down after takeoff from Mexico City, killing 167 people. An overheated brake caused one of the plane's four main tires to burst, with shrapnel severing fuel, hydraulic, and electrical lines. The tire had been erroneously serviced with air instead of nitrogen. According to one summary: "The air, under high temperature and pressure, resulted in a chemical reaction with the tire itself. This led to a chemical explosion of the tire." (The '86 crash is one of the few black marks against Mexicana, one of the world's oldest airlines.)

Inflation pressure too is important. At high speeds, an underinflated tire can generate tremendous amounts of heat. In 1991 a Canadian-registered DC-8 crashed near Jeddah, Saudi Arabia, killing 261 people (it was a charter flight, bringing Hajj pilgrims home to Nigeria). A single, underinflated tire transferred energy to a second tire, and both came apart during takeoff roll. Bits of material then began to burn after gear retraction. Fire spread through the cabin as the plane circled back for an emergency landing, with seats and their occupants ejected through the fuselage.

And as we've been reminded, the fiery crash of an Air France Concorde five summers ago was linked to a fuel tank puncture brought on by an exploding tire.

Exactly what role that tire played, and why it burst in the first place, is the topic of some debate. After weeks of lambasting the media for its distortion-laden aviation coverage, I'm pleased to point readers to David Rose's outstanding analysis of the Concorde disaster. It's not a new piece, written in 2001 for Britain's Observer, but timely to this discussion. Moreover, in addition to his provocative indictment of Air France and the official investigation itself, Rose superbly illustrates how air crashes are rarely the result of a single, definitive cause.

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Do you have questions for Salon's aviation expert? Send them to AskThePilot and look for answers in a future column.


Patrick Smith

Patrick Smith is an airline pilot.

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