F-35 Climatic Testing

By Jeff Rhodes and Lauren Duda Posted 3 April 2015

Neither snow, nor rain, nor heat, nor gloom of night stays these couriers from the swift completion of their appointed rounds.

This famous quote by the fifth century Greek historian Herodotus has been the unofficial slogan of US Postal Service for many years.

But it could also be the motto of the Air Force’s McKinley Climatic Laboratory at Eglin AFB, Florida. With the exception of the “gloom of night” part—the lab’s overhead lights are mostly kept on—that saying sums up what goes on in this one-of-a kind test center in the Florida panhandle.

In the winter of 1942-1943, the German Luftwaffe could not get its aircraft in the air during subzero weather. This reality, coupled with the difficulties the US Army Air Forces Cold Weather Test Detachment was experiencing at Ladd Field, Alaska, made it clear to the US officials that cold weather testing was necessary, and a reliable means of testing had to be found.

The cold weather testing program was officially assigned to the Army Air Forces Proving Ground Command at then-Eglin Field on 9 September 1943. Lt. Colonel Ashley C. McKinley reasoned that testing under controlled conditions would yield far superior test results and would be up to ten times more economical that testing at Ladd Field, which had been expensive and produced only meager results. The solution was to construct a refrigerated hangar at Eglin.

With the completion of the Eglin Climatic Hangar in 1947, the newly born US Air Force acquired its largest and most important test facility. As McKinley played a key role in its design and construction, the Climatic Hangar was renamed in his honor following his death in 1970.

The McKinley Climatic Laboratory, which was extensively renovated in 1997, can recreate nearly every weather condition that exists on Earth with temperatures in the two main test chambers ranging from minus sixty-five to plus 165 degrees Fahrenheit.

An oversized thermometer on the outside of this hangar displays the temperature inside. Although the thermometer is mostly for show, when the needle points to minus forty degrees, the temperature inside really is that cold, even though it might be ninety degrees outside. With its heavily insulated walls and ceiling, the facility is essentially one giant Thermos bottle.

Every major US weapon system and its associated equipment make the trek to Eglin to undergo environmental testing at the lab. The F-35 is the latest to be frozen, baked, deluged, snowed, blown, fogged, and humidified.

Getting Ready
Lockheed Martin test pilot Billie Flynn ferried F-35 test aircraft BF-5 from NAS Patuxent River, Maryland, to Eglin AFB, Florida, in preparation for climatic chamber testing in September 2014. After arriving, the aircraft was prepared for testing and installed in the lab’s 252-foot-wide, 201-foot-deep, and seventy-foot-tall main test chamber.

To accommodate the swiveling engine nozzle and lift-fan system of the Lightning II, a twelve-foot high restraint and support structure interwoven with a system of ventilation ducts was designed. This apparatus secured the F-35 and allowed it to operate at high power in both conventional and short takeoff/vertical landing, or STOVL, mode while inside the building.

The standard press photo for a test subject in the McKinley lab most often shows an aircraft parked and covered in snow. The first photo, from 24 May 1947, shows six aircraft—including a Lockheed P-38 Lightning and a P-80 Shooting Star—in the main chamber.

The photos create an impression that the tests done in the lab are static. In actuality, the tests are almost all highly dynamic, with pilots in the cockpit, maintainers and crew chiefs working in the elements, aircraft systems operating, and engines running.

The lab is one the few facilities in the world where jet engines can be operated in a closed hangar, so air has to get in and exhaust has to get out in order for the engines to run. That air also has to be the same temperature as the test chamber. Otherwise, the results aren’t accurate.

The lab gets that much air from a huge storage tank outside the hangar. The tank provides up to 800,000 cubic feet per minute of either pre-conditioned hot or cold air (depending on the test) to simultaneously replace the air inhaled by the engines. The same amount of air goes in the chamber as comes out.

When the tank is completely emptied, it takes approximately twenty-four hours to replenish the reservoir with conditioned air, particularly with extremely cold or extremely hot air needed for tests in intensely cold or hot conditions.

Going to Extremes
Each of the environmental tests began just like an F-35 flight. The pilot and crew chief would perform a walk-around of the aircraft then the pilot climbed the ladder and strapped into the cockpit—regardless of the conditions in the chamber. The pilot starts the engine and operates various systems on the aircraft in each weather condition.

The aircraft is subjected to all manner of weather extremes in a battery of separate tests. The individual trials range from cold soaking the aircraft at minus forty degrees Fahrenheit; to a variety of icing conditions including freezing rain and ice clouds.

After the snow melts (and is cleared through large drains in the chamber floor to a special retaining pond), heat testing begins. The trials start at a comfortable eighty degrees and build up to 120 degrees with the aircraft’s engine running. A later battery of tests subjects the aircraft to 105 degrees with seventy-four percent relative humidity and 105 degrees with fifty-eight percent humidity to see where condensation occurs inside the aircraft.

During F-35 heat testing, the team simulated a twelve-hour or diurnal day. The engine is shut down, and the temperature in the chamber was steadily increased until it reached it maximum. At that point the aircraft is powered up and the engine started. This test was designed to simulate an pilot starting after it had sat on a hot tarmac all day at a base like MCAS Yuma, Arizona.

Then the rains come. For its final test, the F-35 was taken into the center of a hurricane—so to speak. To gauge how a Lightning II would hold up sitting uncovered on the flight line at a place such as Eglin, the McKinley Climatic Laboratory can be rigged with a system of frames that drench the jet with steady rainfall.

During rain testing, the F-35 was soaked in rainfall ranging from 1.1 inches per hour up to  3.3 inches per hour. Following the steady soak, a spray bar was placed at various positions around the aircraft. A large industrial fan generated sustained winds of forty-four miles per hour in the direction of the spray bar and the F-35, thus creating a tropical storm-like environment.

F-35 testing was completed in March 2015. The aircraft was returned to flight status and then rejoined the Lightning II test fleet at the Aircraft Division test facility at NAS Patuxent River, Maryland.

The McKinley Climatic Laboratory is truly a matchless national resource. With its unique capabilities, there is little wonder why this facility was designated as a National Historic Mechanical Engineering Landmark in 1987.

But the bottom line is practicality. Getting the same kinds of data that can be gathered in only a few months of testing at the laboratory would have taken years without it, and at much greater cost.

Eric Hehs contributed to this article.

Jeff Rhodes is the associate editor of Code One. Lauren Duda is a Communications Department Associate at Lockheed Martin.