Debris Fields

If you’re a news junkie, as I am, in the past 24 hours you’ve probably heard or read about the dangers of airplane debris falling from the sky.

On February 20, 2021, NPR reported that a United Airlines Boeing 777 departing Denver for Hawaii suffered an engine explosion and fire shortly after takeoff. The captain radioed to the Denver tower that they had an “engine failure” and “heavy mayday” as he circled back to land at Denver. The airplane landed safely and there were no injuries.

However, there were dramatic photos and video of enormous pieces of an engine cowling and other parts falling onto a residential neighborhood near the airport. Video of the fire in the remains of the engine taken from inside the passenger cabin are included in this BBC article.

Most people are probably amazed that the 777 could not only continue its takeoff safely with only one engine operating, but also circle back and land safely.

I knew it could. Raised by a Boeing test pilot, and having researched and written about all the tests the FAA requires new commercial jets to pass before being put into service, I was aware that one major proof Boeing (and other manufacturers, like Airbus) must demonstrate is that any twin-engine commercial airplane can take off, fly and land with only one of its two engines operating.

The real marvel isn’t that the 777 landed safely and no one on board was injured, but rather that no one on the ground was hurt by the falling debris.

Reading the news reports and viewing the photo of a major portion of the enormous engine cowling lying in someone’s front yard, I remembered stories my father shared about how every now and again, some part of an airplane they were testing would come loose and fall off from high altitude.

“Wait, what? Really?”  That was my initial reaction when hearing this.

I had no idea.

Nor do most people.

But it happens, more often than one wants to think.

It’s like space debris, there’s lots of it, only in this case, aviation debris comes loose in the lower atmosphere so gravity has its way with it, pulling the debris to the ground. Earth itself becomes an enormous debris field.

Until I learned about regularly-occurring airplane debris from my father, I naively assumed that the only aircraft debris fields one might encounter were the result of crashes. Who knew you had to be wary of pieces of metal falling oh-so-fast toward earth after unexpectedly detaching from an airplane taking off or cruising thousands of feet overhead?

Thankfully, the chances of being hit are minuscule. But…it does happen, as today’s news revealed. This CNN report describes a similar engine failure upon takeoff, this time on a 747-cargo airplane in the Netherlands. And unfortunately, in that case the falling debris injured two people, one seriously, and caused damage to vehicles on the ground. The 747 landed safely at a different, nearby airport.

Given that this topic is making headlines, I thought it might be fun to share a few examples of airplane debris I included in my book, Growing Up Boeing: The Early Jet Age Through the Eyes of a Test Pilot’s Daughter. Keep in mind many of these examples are from decades ago, when both airplane technology and regulations about where test flights were conducted where…looser.

The first and earliest example took place near Wichita, Kansas. In January 1954, Dad was assigned the task of piloting a B-47B to test ejection seats in flight, making sure the seats cleared the airplane’s tail when ejected at high speed. An anthropomorphic dummy was strapped into the copilot’s seat. …These tests were flown from the Boeing test facility at McConnell Air Force Base outside Wichita; the seats were fired over the Smoky Hill bombing range. …During the first test, the seat and dummy were ejected, but the parachute didn’t deploy. …when the dummy was later found on the ground, it was a bit mangled, earning the nickname “Man of Steel.” Pages 48-49.

Early in the jet age (1950s), when testing new commercial jets, a sensing device called a trailing bomb was often used to get a reading of static air pressure outside the body of the airplane. The trailing bomb was a 100-lb device that looked like a torpedo bomb, thus the name. It had a probe to sense static pressure outside the influence of the airplane. The bomb trailed below the airplane, connected by a 125-ft cable surrounded by a pneumatic hose that transferred pressure to transducers located inside the airplane. Occasionally, if a test airplane shed its trailing bomb during testing, a Boeing test engineer had to travel to the test side on a commercial flight, a new trailing bomb in his baggage. Originally, the travel box the trailing bomb was packed into was labeled “Trailing bomb.” This did not go over well with the airlines, so the trailing bomb was renamed “Trailing static device,” which didn’t alarm anyone.” Page 184, footnote. Who knows how many trailing bombs were lost during testing, or where they landed? The thought gives new meaning to the phrase “bombs away!” Similar sensing devices are still being designed and used today.

One critical aspect of testing new commercial jets is making sure they will quickly dampen any vibration that occurs during flight. If a vibration starts and increases, moving through the body of the airplane, it can cause the loss key parts – engines, wings, tail – which could then lead to a crash. These tests are commonly referred to as flutter tests (flutter referring to the vibration), but in truth they’re flutter clearance tests, proving that induced vibrations (flutter) will naturally damp out completely due to the airplane’s design and aerodynamics.

To test for flutter clearance, the test pilots purposefully induce vibrations during flight, in a managed way. One method involves attaching small vanes (a flat metal plate or blade) to wingtips and tail, with an on/off switch in the cockpit that allows the pilots to “excite” the vane, which in turn causes vibration in the wingtip or tail.

The other method involves the pilot using the controls to manually input impulses to the airplane’s wings and other control surfaces. My father described it this way. “You can up an elevator on the tail real sharp to make the airplane bounce and pitch, while you input an aileron real hard, which makes the wings flex up and down. Or you hit the rudder [push against a rudder pedal], which makes the fuselage and engines go sideways. The instrumentation records how rapidly these vibrations or oscillations damp out. If they start to diverge, you are in trouble, and if you don’t back out pretty quickly, you’re in real trouble.” The pilots rely heavily on the engineers on the ground, especially the flutter experts and structural engineers; if they see a problem in the telemetry, they tell the pilots to knock it off pronto. Page 189.

In 1956, Boeing test pilots Dix Loesch and Ray McPherson were conducting early flutter tests on the new KC-135. One particularly hard input from Loesch didn’t damp out as expected. Instead, it amplified until the airplane lost part of its tail.

I thought the airplane was just going to come completely apart,” Dix told me. “I wouldn’t have given a plug nickel for it. The chase pilot thought so, too. He could see the airplane was beginning to come apart. There wasn’t much of the tail left, aerodynamically; it was just hanging there. Then the big decision was where to try to land it. I finally decided I could control it well enough with the engines to land at Boeing Field if I could keep it straight and not excite it. I had to be careful; it was awful goosey.” Page 190. A few pieces of that KC-135 tail didn’t make it back to the airport with the rest of the airplane, though, instead falling somewhere on the ground.

The most dangerous part of flutter testing involves flying the airplane at very high speeds while also inducing a vibration. At higher altitudes the pilots have to put the airplane into a dive in order to achieve the test speed, flying much faster than the airplane would ever be flown by an airline.

According to my father, because flutter-clearance testing is the first time the new airplane is flown at very high speeds, something invariably comes loose – maybe an access panel on an engine strut or a door underneath the airplane opens, or a fairing blows completely off. Engineers are unable to measure air flow loads in the wind tunnel accurately enough to prevent these “mishaps.” Early in his career, Dix was testing B-52s. He remembered one incident when they were climbing up to about 20,000 feet when one of the pressure doors blew off. “Boy, that is a real sensation, a horrible kaPOW! You don’t know what it is. It’s very stunning.” It was just a minor door, however, so losing it didn’t affect flying qualities, Dix assured me.

My father remembered doing flutter-clearance testing on the 707. The NTSB was in charge of documenting and investigating accidents. The rules back then required the filing of a report in any incident that resulted in a certain dollar amount of damage to an airplane, say $250. As you can imagine, building and replacing a panel on a 707 only a foot square in size that blows off would cost way more than $250. “I was flying these flutter flights and almost every flight, I’d lose something,” Dad recalled. “They came off with a bang, like a twelve-gauge shotgun. I was dutifully filing these reports to the NTSB, probably two or three a month, maybe two or three a week in some cases. This guy finally called me up one day and said, ‘Hey, will you quit filing these darn reports. I’ll let you know if I want you to file one.’ It was a lot of work that he didn’t want.”

The 727 was no different. For example, access doors on the belly of the airplane were sucking open at high speeds. The doors had latches on them. The temporary “fix” was to put nut plates in, drill holes and install bolts to hold the doors closed until the tests were done, after which the engineers could design stronger doors and latches.

In case you’re wondering the same thing I was when I heard these stories, these high-speed flights were usually conducted out over the Olympic Mountains or other minimally populated areas. I’ve often fantasized about hiking in the forest on the Olympic Peninsula and coming upon a 707 or 727 access door half buried in the ground like a meteor. They’re out there somewhere. Page 192.

The 777 and 747 news stories of yesterday and today are reminders that these things happen. Not frequently, thankfully, but they’ve been happening since the beginning of aviation, mostly quietly and outside the general public’s awareness. I expect media will report any similar sorts of engine failure and/or debris falling from an airplane for another week or two, but eventually the novelty and newsworthiness will wear off.

One Engine Out

The ability of a commercial airliner to fly with only one engine operating is the other aspect of the United Airlines 777 incident that has featured in news reports.

No doubt, being a passenger on an airplane experiencing an engine explosion during flight, seeing flames in the part of the engine still attached under the wing, would be horrifying, especially if there are only two engines.

It helps, though, to know that twin-engine airplanes are designed to fly safely with just one engine, for long distances if necessary. They are thoroughly tested to prove precisely that ability before being certified and allowed to carry passengers.

Again, from my book Growing Up Boeing:

The other controversy regarding the 767 design was that it had only two engines, yet was designed for long-range routes, such as over the Atlantic. FAA rules originally required that any two-engine passenger airplane, if flying over open water, had to always be within one hour of a suitable airport. This rule was referred to as ETOPs—Extended range Twin Operations. (My father and others at Boeing turned that into the joke, Engines Turn or Passengers Swim.) When the three-engine 727 came along, the rule was waived. That one-hour restriction was gradually loosened to two hours in 1985, after the 767 had been in service a few years, when some airlines wanted to start operations over the Atlantic. Boeing—led by Dick Taylor— was able to convince the FAA that its 767 and 757 had reliable engines as well as reliable electrical, fire suppression, and air pressurization systems, and could operate safely on one engine in an emergency. By 1988, virtually all twin-engine jets qualified for the even less-stringent three-hour limit, opening up nearly the entire planet to twin-engine jet routes, while ringing the death knell for previous three-engine jet designs. Two-engine airliners are much more efficient to operate and maintain. Page 294.

Other Minor Mishaps

Engines aren’t the only part of an airplane that might fail upon takeoff, however.

In the mid-1990s I was seated on a 767, awaiting takeoff for Amsterdam. It was my first – and to date, only – flight on a 767 and I was excited. My boyfriend and I were seated over the left side landing gear, just behind the wing and engine. Charging down the runway and just before the pilot rotated the nose to lift off the runway, I felt a slight jar through my seat and heard a muffled pop. Having listened for years to my father’s stories and having asked tons of questions, I instantly guessed what had just happened: a tire exploded. My boyfriend didn’t notice anything, but I told him, “I think we blew a tire.”

I also knew – thank you, Dad – that this was no big deal. Each side’s rear landing gear had four tires. I thought the captain might say something once we reached cruising altitude, but he didn’t so I second-guessed myself. But eight hours later as we were descending to land at Amsterdam, the captain came on the intercom and advised that because we blew one or more tires at take-off – he wasn’t sure how many – we were going to do a slow, low fly-by past the airport tower with gear down to get a visual report from the controllers, then land on a remote runway. “You’ll see airport firetrucks parked out there,” the captain said, using that calm, reassuring voice all airline captains have mastered in order to keep their passengers from panicking. “Don’t worry. They’re only there as a precaution. I don’t anticipate any issues landing, or any fires.” He had waited until we were descending, seat belts required and tray tables up, before making his announcement, knowing the less time passengers had to think about the situation, the better.

I was reassured, both that I’d been right about the unusual sound and sensation at takeoff, and also that the pilot was aware and doing everything required to land safely. (I assume there was a warning light in the cockpit, alerting crew to the tire issue.) I was certain we weren’t in any real danger. But the aging hippy guy seated behind us who had been drinking and talking loudly the entire flight about how he was going to being playing in a big concert in Amsterdam? He panicked. “I need a cigarette. I need a cigarette!” he repeated, more loudly each time as he put his hands on the back of my seat to pull himself up, getting out of his seat and stepping into the aisle, I presume to go to one of the bathrooms to smoke. A flight attendant had to block him and while being both reassuring and stern, convince him we were going to be fine so he should return to his seat and buckle up. Once back in his seat he kept muttering, “We’re gonna die, we’re gonna die…” as we made final approach.

He was wrong, of course. Dramatic, but wrong. We didn’t die. The landing was smooth. You would never know a tire had blown. I thanked the pilot as we exited the airplane. The worst part of the incident was that because we had been directed to land so far from the terminal, we had to wait for an airport shuttle bus to transport us there and it took forever for our baggage to catch up with us. Aging hippie spent most of that wait time smoking.

A little knowledge can be useful and reassuring, especially when the unexpected occurs.

So, if you’re on a commercial airplane and an engine fails? Don’t panic.

But if you’re near an airport and hear an explosion as an airplane is taking off? Run for cover to avoid being hit by falling debris!

If you’d like to read more stories about the Golden Age of commercial jet aircraft and how Boeing airplanes were tested and certified by a group of dedicated test pilots and flight test engineers, you can find my book Growing Up Boeing: The Early Jet Age Through the Eyes of a Test Pilot’s Daughter here.

Feature image: Lew Wallick inspecting the damage an overheated and exploding wheel brake and tires caused to the fuselage of a 707, late 1950s. Boeing eventually solved the issue by inventing tire valves that released tire pressure upon overheating. Photo courtesy Boeing Archives.