California aeronautical engineer, artist, and pilot Norm Ellis's retrofit, recently approved by the FAA, makes it possible for pilots and passengers to open the side windows of Piper Cherokees, Mooneys, Beechcraft Bonanzas, and other popular aircraft—dozens of models in all—in which collective human sweltering ramps up as the weather warms.
Ellis knows a thing or two about how hot an airplane's interior can get on a sunny day in Southern California, and how being able to open a window on the ground—or, better yet in flight, at altitude—can dramatically improve pilot and passenger comfort. While some general aviation aircraft are still made with windows designed to open in flight, at least on one side, it is often little more than a paltry vent, and the industry has fully fixed in place the windows of many light aircraft—including popular trainers and aircraft otherwise well-suited for travel with family.
Ellis sells the supplemental type certificate online for $500, parts and labor not included, deliverables all in digital form. That includes all required paperwork, and detailed instructions for fabrication and assembly. It's a bit more than an afternoon job, beginning with disassembly of the existing aircraft door and window, and requiring fabrication of a few metal parts that add "negligible" weight: A custom-made metal retainer, hinges, standard fasteners, and a window seal are installed to prevent leaks and hold an open aircraft window securely against aerodynamic loads. (Aircraft owners may opt to find door and window assemblies in a salvage yard from which required parts can be harvested.)
"What you took off is basically what you're putting back on," Ellis said.
The complete installation process requires between 20 and 30 hours of shop time and sheet metal expertise, and all of Ellis's window designs (assembled with original parts and a few added) can safely be opened in flight. Airspeed limitations for open-window operation perplex Ellis somewhat. His openable windows are designed to withstand the aerodynamic forces imposed during level flight at 250 mph.
"I had to design it for a 9-G crash load," Ellis added in a recent telephone interview. The hinges added during the retrofit are rated to withstand 290 foot-pounds of force.
The FAA is nothing if not particular, though the retrofit itself is not, Ellis said, particularly complicated.
Ellis's longstanding love of flying and designing airplanes dates to the late 1950s and early 1960s, riding in his father’s Aeronca Champ, which, notably, had openable windows that were originally designed for the Citabria.
Ellis said the Cessna 182 windows were supposed to remain closed during flight. Drenched and suffering, the young pilot rebelled: "I used to kick the windows open and they would get mad at me for opening the windows."
Years later, during a three-year stint working for the company then known as Mooney International, Ellis would borrow from another decades-old design to develop the control system for the three-seat M10 trainer, and lobby unsuccessfully to equip that aircraft with windows that open. On both sides, ideally.
"This is what I couldn't believe: 'It costs too much money,'" Ellis recalled being told, to which he still responds: "You're trading money for comfort…that doesn't make any sense to me."
Ellis worked at Mooney's aircraft development facility in Chino, California, which the company closed in 2017—around two years after the first flight of the M10 prototype. He came to that job with experience persuading the FAA to approve openable window retrofits, having secured his first openable window retrofit STC in 2001. (His first STC was issued in 1999 for a turbocharger access panel that can be retrofitted to Cessna TP/TU 206, T207, and T210 models, reducing by 20 hours the labor required to access the turbocharger.)
Cessna (now a Textron Aviation company) had been easing out of the open-window game one at a time, preserving the left-side openable window but fixing the right on a growing list of models. The approved model list for Ellis's KG-2 design covers right-hand door openable window retrofits in Cessna 172, 175, 182, 205, P206, 207, and 210 models. A KG-3 variant would follow for the Cessna U206 series windows to open. The most recent AML updates for those early efforts date to 2014.
Ellis explained that a few months of design work and about three years of bureaucratic navigation went into his latest STC, along with the AML that the FAA approved in April, expanding openable window retrofit candidates to include right- or left-side openable window installations in Beechcraft Bonanzas, Mooney M20s, and various Piper Aircraft models including the PA–24, PA–28, and PA–32, among other brands. The earlier KG-2 and KG-3 STCs cover 118 models of aircraft (last updated in 2014), while the NE-1 approved in April adds another 94 models—including many by Beechcraft, Grumman, and Piper.
A few limitations apply: The modification cannot be installed on aircraft with a vent window that is part of the fuselage or door—only on aircraft with a vent window that is embedded in the larger window—and the retrofit cannot be installed in pressurized aircraft.
The NE-1 modification approved in April, very much like the KG-2 and KG-3 before it, is made by disassembling the door and installing fabricated sheet metal parts and fasteners, along with a window seal. Ellis estimates 20 to 25 hours of installation time, along with sheet metal experience, should be required to complete the job.
Ellis said he has sold about 100 openable-window STCs in the roughly 22 years since the first was approved. With a potential population that includes virtually every aircraft in which a CFI can sweat out a pound or two over the course of a summer day of teaching, the majority of the single-engine piston fleet, Ellis started out optimistic that his openable window would provoke more enthusiasm than it actually has.
"I don't know why people don't buy more of them, I really don't," he said. "I've been trying to get this out."
His marketing efforts on social media have been met with bracing skepticism.
"Piper I had to give up on. I went on Facebook…posted in a PA–32 club," Ellis recalled. "They kicked me off."
Ellis said his retrofit is often misunderstood, or dismissed, or unfairly critiqued.
"Some of the comments you wouldn't believe on Facebook," Ellis said. "I am not dealing with engineers."
Ellis said concerns about possible water leaks are not well founded because he specified a tried-and-true tape foam seal between the window frame and opening. "You can't get any water through it." He also noted that the hinged window is built within a frame held in place by a riveted retainer, "so you're not even touching the window opening at all."
A fellow engineer double-checked his calculations before the STC was submitted, Ellis said. "This thing is really structurally sound. "I can kick them open at 140 knots, and they're not going to come off."
Ellis is not waiting around for the GA community to awaken to the fact that opening a window in flight—a standard feature on so many aircraft of yesteryear—is still attainable.
Now three years retired, he is also supplementing his income with visual art, which he exhibits and sells. That gives him a rare and possibly unprecedented distinction among those who have won FAA approval for multiple STCs: "I've been in 216 art shows all over the world."