9 minute read
Gliding Along
MTSU Aerospace Professor Nate Callender adds a potentially life-saving wrinkle to the pilot literature on “engine-out glide”
Article by Drew Ruble
Chesley “Sully” Sullenberger was the US Airways captain who guided his Airbus A320 with 155 passengers aboard to a safe landing in the Hudson River on Jan. 15, 2009, after a flock of Canada geese knocked out both of his airplane’s engines.
The feat, remembered as the “Miracle on the Hudson,” made such an indelible impression that Oscar-winning director Clint Eastwood retold the story in a Hollywood movie titled Sully.
In the film, and in real life, Sullenberger adjusted the plane’s pitch to maintain an optimal glide speed, calculating the correct speed and altitude at which to raise the plane’s nose and slow its descent, creating a softer landing.
Sullenberger’s calm calculations under pressure and his deft touch kept the plane from stalling or dropping hard into the water, maintaining level wings, and kept the plane from flipping or breaking up—outcomes that would have no doubt led to loss of life.
A few avionics companies have developed on-board systems to help pilots navigate “engine-out glides” like what Sullenberger faced.
Existing, in-cockpit displays provide “glide range rings” that inform pilots how far the aircraft should be able to glide at any given moment, as well as the locations of nearby airports. Developed by tech heavyweights like Garmin and Boeing (ForeFlight), avionics software of this type is now an industry standard.
But recently, MTSU professor and associate chair of the MTSU Aerospace Department Nate Callender introduced a way to improve engine-out glide performance that adds a critical new component to the pilot literature.
It could prove to be a life-saving development in future flight.
CRUCIAL CALCULATIONS
Callender made his discovery while working as an expert witness on a court case dealing with a single-engine airplane that lost its engine in flight.
As a part of the court case, Callender used his flight-testing background to design and lead a flight test to determine the aerodynamic performance of this airplane.
He then used that information to make glidepath predictions to the different airports that were available to that airplane at the time of the accident.
In the course of doing those calculations, Callender discovered something novel. Something Callender said pilots don’t know and are not presently taught.
According to Callender, when a pilot loses the engine, making the airplane a glider, the safest landing location may not be straight ahead. A turn will likely be necessary to get there.
How best to turn in an engine-out glide, Callender said, is not taught. Meaning pilots don’t know whether to use a very low bank angle or a really steep bank angle to make the turn.
“I was able to identify the bank angle that gives a pilot the most glide distance when they have to conduct a gliding turn,” Callender said.
“Essentially, current, readily available avionics software does not account for the distance that you would lose if you have to turn in the glide.”
To fill this gap, Callender conducted an optimization of gliding turns to safe landing locations that resulted in a set of equations and associated graphs. This provided the optimum bank angles for different directions that a pilot would have to use in a gliding turn (based upon how far they’d have to turn in an emergency engine-out scenario).
“The equations take into account how high the aircraft is above the ground, its altitude, the glide performance of the airplane (glide ratio), and the amount of turn required,” Callender said. “So, let’s say the safe landing location is directly to your right or left, requiring a 90-degree heading change. Or, if it’s directly behind you, that’s a 180-degree heading change. The inputs into the equation are: how far you have to turn, how high you are when you start, and the airplane’s glide ratio.
“The lower the bank angle, the better your glide ratio will be. But the radius of your turn will be very large, which means you’ll cover more physical distance, and the whole time you’re losing altitude. So, you steepen the bank angle, which decreases the glide radius and minimizes the time that you’re descending, and it puts you on an intercept course sooner, allowing you to level your wings and get back to the airplane’s wings-level glide ratio.”
WHEN A PILOT LOSES THE ENGINE, MAKING THE AIRPLANE A GLIDER, THE SAFEST LANDING LOCATION MAY NOT BE STRAIGHT AHEAD.
Callender said avionics companies could incorporate this information into their software. In the interim, pilots can add this information to their knowledge base.
“It would be a lot to expect a pilot in the cockpit to remember an equation and to do the mental math with complex equations, even on a calculator, in an emergency,” Callender said. “However, you could expect them to know that the optimum bank angle is a variable that you need to think about if you have to turn to different locations in an engine-out glide. And for the most part, the more turn that’s required, the steeper the bank angle needs to be.”
Callender has published this information in multiple places, as well as presented his results at various conferences including at the National Association of Flight Instructors (NAFI), the University Aviation Association, the Society of Flight Test Engineers, and the National Training Aircraft Symposium, a conference attended by a mix of practical aviation trainers and academic departments like MTSU’s.
NAFI president Paul J. Preidecker said Callender’s recent presentation at the NAFI annual conference was very well received.
“Dr. Callender presented detailed mathematical data into easy-to-understand conclusions and recommendations for best practices. His focus on safety and risk mitigation aligned very well with the conference,” Preidecker said. “Dr. Callender offered to send attendees his Glide Optimization Tool, which is a practical, easy-to-use tool to help determine glide performance in an engineout situation. Several pilots requested this information.”
Representatives of both Boeing and Garmin were in attendance at Callender’s presentation to engineers. He has encouraged each company to consider this potential enhancement to their software.
AVIONICS COMPANIES COULD INCORPORATE THIS . . . INTO THEIR SOFTWARE. IN THE INTERIM, PILOTS CAN ADD . . . TO THEIR KNOWLEDGE BASE.
A LIFE IN GLIDE
That Callender would discover something new regarding engine-out glide should come as no surprise given his background.
Callender’s first experience as an actual pilot was not in a plane but in an unpowered glider, learning to stay aloft and to monitor the air and ground for potential sources of lift—whipping winds moving upward in waves.
Callender would eventually become a pilot, earning his master’s in Aviation Systems and his Ph.D. in Engineering Science from the University of Tennessee Space Institute. Prior to joining academia, he worked for the Army technical test center in Fort Rucker, Alabama, doing flight testing for the Army primarily in helicopters (the attack division, responsible for flight tests on Apaches, Blackhawks, Chinooks, and others).
Even to this day, though, gliding is a big part of his life—and academic pursuit.
For the last 20 years, one of the courses that he regularly teaches is Aircraft Performance. Glide performance is a portion of the curriculum.
“In my opinion, it’s one of the most important classes because glide performance is what pilots need to think about when things go wrong,” Callender said. “Because in airplanes, especially single-engine airplanes, like what most of our students learn in at MTSU, if you lose the engine, you become a glider.”
Callender also teaches a course called Theory of Flight, teaching the basics behind how aircraft fly, how engines work, and the math and physics involved. He even wrote a grant for that course that enabled him to begin providing students hang-glider training, which he taught near Lookout Mountain in southeast Tennessee.
Callender’s fascination with gliding perhaps stems from what he describes as his earliest aviation memory.
“I had a dream when I was a kid where I was running on the beach and I put my arms out and I took off and I was just flying over the beach,” he said.
Born and raised in west Tennessee in a little town called Halls, Callender’s first actual taste of flight came when he was just 2 years old.
He doesn’t remember it, but his family told him the story. His hometown was home to an abandoned World War II Army air base. It still has one active runway you can fly into today.
Callender’s grandparents and great-grandparents both lived near the property where the air base used to be. They had farmland there, and one of the abandoned concrete runways basically was the road that went past the front of their houses.
There was a missionary pilot who flew in when Callender was 2 years old and gave him a ride in his airplane.
“That’s what I’ve been told is my first airplane experience,” Callender said. “But from a young child, I’ve always liked and thought about aviation.”
These days, Callender’s thinking about aviation is leading to enhanced safety measures.
Air travel keeps getting safer, according to a study by MIT researchers. The risk of a fatality from commercial air travel was 1 per every 13.7 million passenger boardings globally in the 2018–22 period—a significant improvement from 1 per 7.9 million boardings in 2008–17 and a far cry from the 1 per every 350,000 boardings that occurred in 1968–77, the study found.
Nevertheless, safety improvements in flight are always welcomed, and commonly stem from new technologies.
Research like Callender’s has the potential to lead to technological advancements that improve aviation performance—and save lives.
[Editor’s note: Skip Anderson contributed heavily to this report].
