aviation – Close Read

The new and large fly the farthest

British Airways and Air France mutually retired the Concorde supersonic jet in 2003. Both companies cited rising maintenance costs as being the reason, which in turn were compounded by falling demand after the Paris crash in 2000 and a general downturn in civil aviation after 9/11. Now, American and French scientists have found that Concorde was in fact an allometric outlier that stood out design-wise at the cost of its feasibility and, presumably, its maintenance. Perhaps it grounded itself.

One thing Adrian Bejan (Duke University), J.D. Charles (Boeing) and Sylvie Lorente (Toulouse University) seem to be in awe of throughout their analysis is that the evolution of commercial airplane allometry seems deterministic (allometry is the study of the relationship between a body’s physical dimensions and its properties and functions). This is awesome because it implies that the laws of physics used to design airplanes are passively guiding the designers toward very specific solutions in spite of creative drift, and that successive models are converging toward a sort of ‘unified model’. This paradigm sounds familiar because it could be said of any engineering design enterprise, but what sets it apart is that the evolution of airplane designs appears to be mimicking the evolution of flying animals despite significant anatomical and physiological differences.

One way to look at their analysis is in terms of the parameters the scientists claim have been guiding airplane design over the years:

Wingspan
Fuselage length
Fuel load
Body size

Among them, fuel load and body size are correlated along the lines of Tsiolkovsky’s rocket equation. It says that, for rockets, if two of the following three parameters are set, the third becomes immovably fixed in a proportional way: energy expenditure against gravity, potential energy in the propellant, and the fraction of the rocket’s mass made up by the propellant. According to Bejan et al, there is a corresponding ‘airplane equation’ that shows a similar correlation between engine size, amount of fuel, and mass of the whole vehicle. The NASA explainer finds this association tyrannical because, as Paul Gilster writes,

A … rocket has to carry more and more propellant to carry the propellant it needs to carry more propellant, and so on, up the dizzying sequence of the equation

Next, there is also a correlation between wingspan and fuselage length corresponding to an economy of scale such as what exists in nature. Bejan et al find that despite dissimilarities, airplanes and birds have evolved similar allometric rules on the road to greater efficiency, and that like bigger birds, bigger airplanes are “more efficient vehicles of mass”. Based on how different airplane components have evolved over the years, the scientists were able to distill a scaling relation.

S/L ~ M^1/6 g^1/2 ρ^1/3 σ^1/4(ρ_aV²C_l)^-3/42^1/4C_f^7/6

Be not afraid. S/L is the ratio of the wingspan to the fuselage length. It is most strongly influenced by ρ_a, the density; σ, the allowable stress level in the wing; g, the acceleration due to gravity; and C_f, the fixed skin-friction coefficient. More interestingly, the mass of the entire vehicle has a negligible effect on S/L, which pans out as a fixed S/L value across a range of airplane sizes.

Citation: J. Appl. Phys. 116, 044901 (2014); http://dx.doi.org/10.1063/1.4886855

Similarly, the size of a plane’s engine has also increased proportional to a plane’s mass. This would be common sense if not for there being a fixed, empirically determined correlation here as well: M_e = 0.13M^0.83, where M_e and M are the masses of the engine and airplane, respectively, in tons.

During the evolution of airplanes, the engine sizes have increased almost proportionally with the airplane sizes (the data refer only to jet engine airplanes). J. Appl. Phys. 116, 044901 (2014); http://dx.doi.org/10.1063/1.4886855

In terms of these findings, the Concorde’s revolutionary design appears to have been a blip on the broader stream of traditional yet successful ones. In the words of the authors,

In chasing an “off the charts” speed rating the Concorde deviated from the evolutionary path traced by successful airplanes that preceded it. It was small, had limited passenger capacity, long fuselage, short wingspan, massive engines, and poor fuel economy relative to the airplanes that preceded it.

That the Concorde failed and that the creative drift it embodied couldn’t achieve what the uninspired rules that preceded it did isn’t to relegate the design of commercial airplanes to algorithms. It only stresses that whatever engineers have toyed with, some parameters have remained constant because they’ve had a big influence on performance. In fact, it is essentially creativity that will disrupt Bejan et al‘s meta-analysis by inventing less dense, stronger, smoother materials to build airplanes and their components with. By the analysts’ own admission, this is a materials era.

Bigger airplanes fly farther and are more efficient, and to maximize fuel efficiency, are becoming the vehicles of choice for airborne travel. And that there is a framework of allometric rules to passively maximize their inherent agency is a tribute to design’s unifying potential. In this regard, the similarity to birds persists (see chart below) as if to say there is only a fixed number of ways in which to fly better.

The characteristic speeds of all the bodies that fly, run, and swim (insects, birds, and mammals). J. Appl. Phys. 116, 044901 (2014); http://dx.doi.org/10.1063/1.4886855

From the paper:

Equally important is the observation that over time the cloud of fliers has been expanding to the right . In the beginning were the insects, later the birds and the insects, and even later the airplanes, the birds, and the insects. The animal mass that sweeps the globe today is a weave of few large and many small. The new are the few and large. The old are the many and small.

References

The evolution of airplanes, J. Appl. Phys. 116, 044901 (2014); DOI: 10.1063/1.4886855

The stuff we learn after a plane goes missing

(A version of this post, as written by me, first appeared in The Hindu science blog, The Copernican, on March 16, 2014.)

It’s likely any of you knew many of or all the following, but these are things I became aware of from reading news items and analyses of the missing Malaysian Airlines flight 370, currently one of hijacked, crashed into a large water-body or next-plausible-occurrence. While some of them may not directly apply to the search for any survivors or the carrier, all of them shine important and interesting light on how things work.

Ringing phones aren’t actually ringing. Yet. – After the relative of a passenger on board flight 370 called up the person’s phone, it started to ring. This was flashed on TV channels as proof of the plane still being intact, whether or not it was in the air. A couple hours later, some telecom experts wrote in that the first few rings you hear aren’t rings that the call’s receiver is hearing, too. Instead, those are the rings the network relays to you so you don’t cut the call while it looks for the receiver’s device.

Air-traffic controllers don’t always know where the plane is* – Because planes are flying at 35,000 feet, controllers don’t anticipate much to happen to them, and they’re almost always right. This is why, while cruising at that altitude, pilots don’t constantly buzz home to controllers about where their flight is, its altitude, its speed, etc. To be on the safe side, they buzz home over specific intervals, a process that’s automated on some modern models. Between these intervals, of course, the flight might just as well be blinking in and out of extra dimensions but no one is going to have an eye on it.

Radar that controllers have access to don’t work so well beyond a range of 150-350 km** – If civilian aircraft are farther than this, they no longer show up as pings on the scanning screen. In fact, in another system, called automatic dependent surveillance-broadcast (ADS-B), a plane determines its location based on GPS and transmits it down to a controller. Here again, there’s a distance limit of up to 300 or so km. Beyond this, they communicate over high-frequency radio. Of course, this depends on the quality of equipment, but it’s useful to know such limitations exist.

If a plane’s communication systems have been disabled, there’s no Plan B – There’s radar, then radio, then GPS, then a fourth system where the aircraft’s computers communicate via satellite with the airline’s offices. The effectiveness of radar and radio is contingent on weather conditions. Beyond a particular altitude and, again, depending on the weather, GPS is capable of blinking out. The fourth system can be be manually disabled. If a renegade technician on the flight knows these things and how to work them, he/she can take the flight off the grid.

For pilots, it’s aviate, navigate, and then communicate – If the flight is in some kind of danger, the pilot’s primary responsibility is to do those things necessary to tackle the threat, and try and get the carrier away from the danger area. Only then is he/she obligated to get in touch with the controllers.

The ocean is a LARGE place – Sure, we studied in school that the oceans cover 71% of Earth’s surface and contain 1.3 billion cubic km of water, but those were just numbers – big numbers, but numbers nonetheless. I think our sense of bigness isn’t reliant any bit on numbers but only on physical experiences. I’m 6’4″ tall, but you’ll have to come stand next to me to understand how tall I really am. That said, I now quote former US Navy sailor Jim Wright (from his Facebook post):

… even when you know exactly, and I mean EXACTLY, where to look, it’s still extremely difficult to find scattered bits of airplane or, to be blunt, scattered bits of people in the water. As a navy sailor, I’ve spent days searching for lost aircraft and airmen, and even if you think you know where the bird went down, the winds and the currents can spread the debris across hundreds or even thousands of miles of ocean in fairly short order. No machine, no computer, can search this volume, you have to put human eyeballs on every inch of the search area. You have to inspect every item you come across – and the oceans of the world are FULL of flotsam, jetsam, debris, junk, trash, crap, bits, and pieces. Often neither the sea nor the weather cooperates, it is INCREDIBLY difficult to spot [an] item the size of a human being in the water, among the swells and the spray, even if you know exactly where to look – and the sea conditions in this part of the world are some of the worst, especially this time of year.

Mr. Wright goes on to write that should flight 370 have crashed into the Bay of Bengal, the South China Sea or wherever, its leaked fuel wouldn’t exactly be visible as an oil slick because of two reasons: first, high-grade aircraft fuel evaporates really fast (if it hasn’t already been vaporized on its way down from the sky); second, given the size of the fuel-tank, such a slick might cover a few square kilometers: on an ocean, that’s a blip. The current extended search area spans 30,000 sq. km.

Military threats in militarized zones are discerned by ballistic trajectories of bodies – One of the simplest ways armored units know what they’re seeing in the sky is not a missile but a civilian aircraft is by their trajectory – the shape of their path. Most missiles are ballistic, which means their trajectories are like upturned Us. Aircraft, on the other hand, fly in a straight line. I suppose this really is common sense but it is good to know just what’s keeping me from getting bombed out of the air should I fly over, say, the East China Sea…

The global positioning system doesn’t continuously relay the aircraft’s location to controllers – See * and **.

Smaller nations advance pilots with fewer flying hours than is the norm in bigger nations – According to a piece on CNN, one of flight 370’s two pilots had clocked only 2,763 flying hours as a pilot, and was “transitioning from flight simulator training to the Boeing 777-200ER”. The other pilot had a little over 18,000 hours under his belt. As CNN goes on to explain, smaller nations tend to advance pilots they think are very talented, farther than they could go in the same time in other countries, through intensive training programs. I couldn’t find anything substantive on the nature of these supposedly advanced programs, so I can’t comment further.

Pilot suicide – Okay, what the hell. Nobody wants a person at the controls who’s expressed suicidal tendencies, and it’s the airline’s responsibility to treat or accordingly deal with such people. However, the moment you’ve said that, you realize how difficult such situations could be to predict, not to mention how much more difficult to prevent. A report by the US Federal Aviation Administration titled ‘Aircraft-Assisted Pilot Suicides in the United States‘, from February 2014, describes eight case-studies of flights whose pilots have killed themselves by crashing the aircraft. Each study describes the pilot’s behavior during the flight’s duration and is careful to note no other electric/mechanical failures were present. In the case of flight 370, of course, pilot suicide is just a theory.

The Boeing 777 is one safe carrier – Since its first flight in 1994, the Boeing 777-200ER (for ‘Extended Range’) had an estimated full loss equivalent (FLE) of 0.01 as of December 31, 2012, over 6.9 million flights. According to AirSafe.com, the FLE…

… is the sum of the proportions of passengers killed for each fatal event. For example, 50 out of 100 passengers killed on a flight is an FLE of 0.50, 1 of 100 would be a FLE of 0.01. The fatal event rate for a set of fatal events is found by dividing the total FLE by the number of flights in millions.

The same site also lists the 777-200ER as having the second lowest crash rate – 0.001 per million flights – of all time, among all models with 2 million flights or more, as of September, 2013. Only the Airbus A340 is better with a crash rate of 0, although it has clocked 4 million fewer flights (just saying).

Southeast Asia is a busy area for aviation – Between April-2012 and October-2013, the number of seats per week per Southeast Asian country grew by an average of 19.4%. In the same 18 months, the entire region’s population grew by 6% (both numbers courtesy the Center for Asia-Pacific Aviation). Then, of course, there’s Singapore’s Changi Airport. It’s one of Asia’s busiest, if not the world’s, handling 6,100 flights a week. And it was in this jam-packed area that people were trying to look for one flight.

For more on how we can manage to lose a plane in 2014, check out my previous post Airplanes Can Still Go Missing.

Airplanes can still go missing

Airplanes are one of our largest modes of transportation in terms of physical size. With the exception of ships, airplanes have the highest carrying capacity, are quite environmentally disruptive while in operation, and are equipped with some of the most sophisticated positional tracking technologies.

Yet, one still went missing last week. Fourteen years into the 21st century, while the NSA threatens the privacy of global telecommunications, one airplane goes missing. I don’t mean to trivialize the issue of the Malaysian Airlines Flight 370 turning untraceable, but just that even though some of us are smart enough to build invisibility cloaks, we also still have our problems.

I went around the web trying to understand why this was the case and found some interesting stuff. Even though #370 was only the third flight to go missing in the 21st century (and almost the 1,900th to have crashed), it is the 111th flight to do so since radio-sets were first installed on airplanes in 1917. On average, that’s a little more than one disappearance per year.

One reason finding missing airplanes is so difficult is multiplicity. Airplanes are made up of thousands of components each. When one component malfunctions, it could lead to a form of failure that’s very different from what would happen when a different component malfunctions. Watch an episode of Air Crash Investigation on National Geographic if you don’t believe me—airline investigators trying to figure out what exactly could have wrong often find the blame lies with small deviations from normal practices by the pilots or maintenance crews. I remember an episode titled Disaster on the Potomac that aired in December 2013, which details how the 1982 Air Florida crash that killed 78 people was due to a faulty de-icing procedure that skewed instrument readings in the cockpit.

On top of this, you have environmental factors to deal with. According to a piece by Jordan Golson in Wired on March 11, Col. J. Joseph, an aviation consultant, thinks that when planes break up at higher altitudes, the debris is likely to be moved around by stronger winds. Given that flight #370 was over 11 km up, Col. Joseph thinks windspeed could have been over 180 km/hr, enough to blow pieces out of any geographical context.

Things get worse if the plane crashes into the water. Consider the oft-quoted example of Air France Flight 447, which crashed into the Atlantic Ocean in 2009 with 228 people on board. Rescue missions took almost two days to find the first signs of its wreckage. Before that, in 2007, an Indonesia Boeing 737 crashed near the Makassar Strait near Sulawesi. Its wreckage took 10 days to find. There are many other sad yet interesting examples.

According to a Wall Street Journal analytical piece by Daniel Michaels and John Ostrower on March 11, the search for #370 could be further hampered by the fact that the region it was traversing is one of the busiest on the planet: Southeast Asia. Moreover, according to Golson, radar isn’t good enough after the plane’s farther than about 200 km from the nearest control tower, while precise GPS locations aren’t relayed continuously by the pilots to air-traffic controllers—this is why we rely on a ‘last known location’, not a definitive ‘last location’. At the same time, controllers don’t panic when pilots don’t ping back frequently because, according to pilot Patrick Smith’s blog:

In an emergency, communicating with the ground is secondary to dealing with the problems at hand. As the old adage goes: you aviate, navigate, and communicate — in that order. And so, the fact that no messages or distress signals were sent by the crew is not surprising or an indicator of anything specific.

However, what’s stranger about flight 370 is that it’s a Boeing 777, which comes with an emergency locator that beeps out location signals for many days after a crash. Rescuers are yet to spot one in the area they’re combing. So, as the search for a missing airplane drags on, it’s only our conviction that some trace of the vehicle will surface that lasts, accompanied by stronger and stronger scrutiny of what facts we manage to gather (In the meantime, the Daily Mail has something about an aeronautical black hole you might want to read about).