ext weather

"Extreme environmental testing of weapon systems is a vital necessity in any global military effort," wrote Clendon L. Hendrickson in the 1988 handbook Flight Testing Under Extreme Climatic Conditions. Today, the words of the then US Air Force Airframe Systems Division's deputy chief are, if anything, even more relevant, as operational theatres continue to expand and aircraft become increasingly complex.

With military aircraft now likely to experience most, if not all, of the worst of the world's climatic extremes over the course of their service life, understanding how and where failures may occur is the key to ensuring reliability and so maintaining operational capability in the field.

The problem with the weather is, however, that it is essentially unreliable and that can play havoc with test programmes. Environmental conditions cannot be controlled during deployments to actual real-world test sites, and if the desired climatic events simply do not occur as expected, it will often be impossible to attempt testing again until the following year, when the appropriate season comes around again.

However, there is an alternative to chasing the weather: it can be simulated. At the forefront of this is the McKinley Climatic Laboratory (MCL), a modern, high-tech testing facility with a surprisingly long history of helping the US Air Force (USAF) keep its aircraft operational, whatever nature throws at them.

The world's largest climatic test facility

"The McKinley Climatic Laboratory is the world's largest climatic test facility – large enough to test a C-5 Galaxy," says MCL chief Dwayne Bell.

Within its 5,100m2 of operating space it can replicate just about any weather conditions on the planet except, Bell points out, lightning strikes – although this can be done at another USAF facility – or tornadoes. Such limitations aside, within the confines of the MCL at Florida's Eglin Air Force Base, it can rain at a rate of 380mm/hour, the wind can blow at 60 knots, sand and dust storms can rage, salt fogs be made, snow fall, ice form and the temperature range from 74oC down to -54oC.

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It puts the laboratory in a completely different league from other much smaller, typically room-sized, facilities capable of testing cars and trucks, or even the handful of others that are large enough to accommodate small aircraft, but which cannot test them with their engines running or rotors turning. Moreover, it enables MCL to support a wide range of military and civilian aircraft programmes – both the Boeing 787 and the Airbus A350 XWB were tested here – as well as a long list of others including cars, trucks, tanks, missile launchers, shelters and engines.

"For most of our military and commercial aircraft customers, they only have two options; MCL or chase the weather," says Bell.

Getting better results with controlled conditions

The US military's involvement with climatic testing dates back to a recommendation made in 1934 which ultimately saw the Ladd Field in Fairbanks being designated a test base in 1940. Regular cold weather testing began two years later.

By 1944, however, it became apparent that this approach was inconvenient, expensive and provided fewer results than had been expected. It led Lieutenant Colonel Ashley McKinley, who knew the importance of cold weather reliability from his wartime experience ferrying aircraft to the Soviet Union, to propose the construction of a refrigerated hangar at the Eglin base, to reproduce the Alaskan climate under controlled conditions.

He reasoned that this approach would yield more useful results, with greater reliably and ultimately at around a tenth of the cost. Although the construction costs were to spiral up to nearly three times the original $2m estimate before testing was finally able to get underway in 1947, Hendrickson records that the actual savings realised subsequently have proved to be not far adrift of McKinley's original prediction.

From sandstorms to solar radiation

From this original, giant refrigerated hangar, 78mlong by 75m wide and 21m high, MCL has grown over the years to add five other specialised testing facilities – a sun, wind, rain, and dust chamber, salt fog chamber, all-weather room, temperature-altitude chamber and the equipment test chamber.

Simulating the range of climatic conditions within them relies on a number of technologies and tools. Large ducted fans provide the wind, capable of blowing from gentle breeze to hurricane force. The wind also often forms part of other tests, using additional elements such as artificially created snow and rain, or commercially available sand or silica flour to mimic sand and dust storms.

"Large arrays of heat lamps suspended over the aircraft allow the thermal effects of solar radiation to be investigated."

Rain is simulated by water spray nozzles suspended over the test aircraft, or arranged vertically in a wind stream for tests involving possible water intrusion, with the rate and droplet size carefully controlled by adjustments of the flow rate and nozzle size, while commercial snow machines provide the snow. At the other end of the climatic spectrum, large arrays of heat lamps suspended over the aircraft allow the thermal effects of solar radiation to be investigated, with 72 settings between fully on and fully off enabling a very realistic 24-hour day/night cycle to be reproduced within the chamber.

Monitoring performance

Before any of this can be used, however, MCL staff first needs to design, fabricate and install a restraint system which is specific to each individual aircraft and approved by the manufacturer's own structural engineers. Load cells are often incorporated into this, to enable MCL engineers to monitor the physical forces in the restraint system to ensure that the aircraft's own structural loading limits – provided by the original manufacturer – are not exceeded.

Bell says that sometimes the test subjects come fully instrumented, complete with their own data acquisition system, so that MCL only needs to display and record the appropriate weather conditions such as temperature, humidity, solar radiation or icing cloud droplet size. At other times there is no instrumentation pre-installed and MCL engineers will then install suitable systems to record whatever test parameters that are needed.

In addition, almost every test makes use of the facility's high-definition video monitoring system, which can provide overall views from the chamber walls, while tripod-mounted cameras can be used to record events from anywhere within the chamber – or even from inside the aircraft itself.

The team does not, though, have a role in actually evaluating the performance of the aircraft or components being tested. As Bell explains, "our job is to create the weather; our customers have their own test engineers and maintainers on site to conduct the testing – or maintain and repair the item if it fails."

Of course, the ultimate yardstick for any testing regime has to be how closely performances under artificial conditions match up to actual performances in the real world. While MCL again does not itself do any field evaluations of test items, it seems that the feedback received over the years indicates that the data collected at the facility correlates extremely well with the corresponding results for natural weather conditions.

"MCL works very hard to ensure that all weather simulations are as accurate as possible," Bell says. "Our artificial weather cannot be 'better' than Mother Nature; we can only strive to be equivalent."