Since it first became an active cold-weather testing facility in 1947, the 96th Test Wing's McKinley Climatic Laboratory at Eglin Air Force Base, FL, there have been a wide variety of aircraft to undergo testing at the facility, ranging from the B-29 Superfortress and P-51 Mustang through to the Lockheed F-117, Boeing 787, and Airbus A350 XWB.
Most recently, one of Lockheed Martin’s F-35Bs from the F-35 Patuxent River Integrated Test Force in Maryland underwent rigorous climatic testing at the laboratory to verify its all-weather capabilities on its way toward Initial Operating Capability (IOC). The F-35B arrived at McKinley in September 2014, to begin a six-month assessment of the aircraft's performance in wind, solar radiation, fog, humidity, rain intrusion/ingestion, freezing rain, icing cloud, icing build-up, vortex icing, and snow.
With 13 countries currently involved with the program, the F-35 must be tested in all the meteorological conditions representative of those locations from which it will operate, ranging from the heat of northern Australia to the bitter cold of the Arctic Circle above Canada and Norway.
Testing for the F-35 can be done from -40° to +120°F “and every possible weather condition in between," said Billie Flynn, an F-35 test pilot who performed extreme cold testing on the aircraft. "It has flown in more than 100°F heat while also flying in bitter subzero temperatures. In its final days of testing, it will fly through ice and other conditions such as driving rain with hurricane force winds. We are learning more and more about the aircraft every day.”
The chamber allows for the simulation of “virtually any weather condition—all while flying the jet at full power in either conventional or vertical takeoff mode," said Dwayne Bell, the McKinley Climatic Laboratory technical chief.
As the F-35 approaches its IOC debut for the U.S. Marine Corps this year, testing for the STOVL (short takeoff/vertical landing) variant required a few more adaptations to procedures than aircraft before it. Eglin AFB reports that the lift-fan system of the F-35B required the design of a 12-ft high “restraint and support” structure interwoven with a system of ventilation ducts. This apparatus secures the aircraft and allows it to operate at high power in both conventional and STOVL mode while inside the building.
To ventilate the exhaust and thus maintain a stable temperature inside the chamber, conditioned air is constantly pumped in to ensure the pressure in the building is always higher than the pressure inside the ducts surrounding the engine and other openings on the aircraft. This difference in pressure is a safeguard that maintains the jet exhaust is flowing out of the chamber through the ducts, allowing the facility to sustain a constant temperature.
Over days of high-temperature climatic testing, the temperatures of the engine runs were steadily and incrementally increased until it reached the test maximum of 120°F. While the chamber itself was set to a pre-determined temperature, additional solar lamps above the aircraft recreated the intense heat of the sun on the surface of the jet. This is done for the obvious reason that air temperature does not remain constant throughout the day—it increases each hour the sun is up, reaching its apex in the late afternoon. In the chamber, engineers recreated the temperature fluctuation of a 24-h day.
In a matter of days, the chamber transitioned from a seemingly Arizona sauna to the Arctic Circle. Outside air was super-cooled using McKinley Lab’s refrigeration system and pushed into the chamber. In increments, the chamber temperature fell to -40°F while the jet completed test runs along the way. At such frigid temperatures, aircraft fluids start to thicken and mechanisms operate slower—all points test engineers monitor closely.
The F-35 then faced a harsh barrage of snow and ice. When an aircraft flies through clouds at high speeds in freezing climates, large pieces of ice can form quickly on the exterior. Heavy chunks of ice could potentially break off and damage the aircraft, errantly fly into the engine or create a foreign-object-damage concern. For this reason, icing is one of the most dangerous elements in climatic testing.
A large apparatus composed of three massive cylinders stacked in a pyramid, parallel to the ground, was placed in front of the F-35 test aircraft. Inside the cylinders were nine ducted fans that blew a large amount of air through a single funnel in the front. Attached to the front of this funnel was a spray bar capable of producing “clouds” of various water droplet sizes.
Those droplets were blown toward the plane and froze upon contact. While the lab cannot generate the precise wind speeds experienced by airborne aircraft, the unique setup inside the chamber is capable of producing sustained wind speeds up to 120 mph—all while subjecting the F-35 to precipitation.
Snow and ice testing gauges the effectiveness of the aircraft’s Ice Protection System and the ability of the jet to perform in winter weather.