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September 2018

Amazing and astonishing flight with research Fly-By-Wire system

by honored guest writer Masayuki Sato

Mr. Sato is an Associate Senior Researcher at the Aeronautical Technology Directorate of the Japan Aerospace Exploration Agency (JAXA). JAXA is Japan’s national research and development agency, created in 2003 following the merger of three institutions: the Institute of Space and Astronautical Science (ISAS), the National Aerospace Laboratory of Japan (NAL) and the National Space Development Agency of Japan (NASDA). JAXA addresses a wide range of research activities both in aeronautics and space. JAXA creates, matures, and demonstrates advanced aeronautical technologies to make air transportation systems safer and greener.

One of the most fundamental facilities for aeronautical R&D is the research aircraft. JAXA has one rotorcraft (a BK117 C-2) and two airplanes; the Cessna 680 Citation Sovereign and the Dornier 228-202. At JAXA, we firmly believe that verification by flight demonstration is essential to innovation in aerospace science. Therefore, we are continuously conducting a variety of flight tests to research potential new solutions as well as find new questions to ask and new challenges to resolve.

A Dornier 228-200 has been used as a research airplane at NAL in Japan since 1988. To enhance the Dornier 228’s ability as a research airplane, we implemented an experimental Fly-By-Wire (FBW) system in 2000. In the course of this renovation, we replaced the original right-seat control column, wheel, pedal and power levers with artificial ones so that they were not directly linked to the original mechanical linkage. These artificial devices produce only electric signals, which are sent to the FBW computer. The new-born Dornier 228 is now called the “Multi-Purpose Aviation Laboratory-α”, in short “MuPAL-α”. My boss created this name and the “α” denotes the first letter of “αεροσκάφους” which means “airplane” in Greek. I thought to myself, most people couldn’t pronounce this, and with a smile I have to say I now doubt the nickname sense of my boss!

The most impressive and important feature of MuPAL-α is the freedom of the onboard FBW program. Of course, for safety reasons, we must strictly follow the regulated rules, but within the rules, we can change the program as we like! It is almost like a toy for adults, but a very expensive one! Thanks to this unique feature, we could conduct a wide variety of flight tests. I remember one very particular test experience. This test gave us the most impressive and amazing (to be honest, also astonishing and frightening) experience among all our previous tests.

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Back left view of the MuPAL-α during preparation for Horizon2020 flight tests, Japan 2015. (Photo: JAXA)

An In-Flight Simulator (IFS) is an aircraft that mimics the characteristics of other aircraft. The experimental FBW system plays a key role for the aircraft to be an IFS. We first designed (sophisticated, I hope) flight controllers which have capabilities to mimic other aircraft dynamics. Then, we implemented our flight controller within the experimental FBW system by creating the C-code ourselves. Finally, the MuPAL-α could fly as an IFS when we engaged our newly designed flight controller. However, we are human-beings, thus we sometimes (only “sometimes”, I believe) make mistakes in the C-code. When this happens, unexpected motions sometimes are driven by our C-code.

And so, it happens one exciting day

It is a well-known fact that the signs of control devices (aileron, rudder and elevator) can be different from country to country. We indeed know it! But, due to our silly mistake, the sign of aileron was wrongly set and we didn’t realize it when writing the C-code. Furthermore, we also did not realize it during the on-ground tests. We usually conduct on-ground tests to check the normality of our flight controller as well as our C-code. Passing the normal pre-flight check, MuPAL-α (with its problematic flight controller) took off from Chofu airport and flew to our test area. We followed the well-established procedure: we first conducted the FBW normality check by engaging “228 mode” (this mode produces the same control commands as the conventional control column, wheel, pedal and power levers) and concluded that there were no faults in the FBW system. We then checked the function of the “disengage button” by engaging and instantaneously disengaging the FBW experimental mode. This function is crucial for safety; if some abnormality is found then the safety pilot sitting in the left-hand seat pushes the “disengage button” and is able to recover the conventional mechanically linked control system.

Up to this point, no faults had been found. We proceeded to the next step, which was a flight test with our flight controller engaged. As soon as our flight controller was engaged, the evaluation pilot sitting in the right-hand seat said “the wheel action is opposite!” At first, we couldn’t understand what he said, because we had found no faults during the on-ground check. So, we engaged our flight controller again after persuading the evaluation pilot. And again, the evaluation pilot screamed: “The wheel action is opposite!” So, we stood up from our seats and checked the motions driven by the right-hand wheel. At last, we found the problem! Indeed, the clockwise maneuver was creating counter-clockwise roll motions, and the counter-clockwise maneuver was creating clockwise roll motions. At this moment, we realized that we had made a silly mistake and that the signs of the ailerons were mistakenly set in our C-code.

Even with this excitement, and a big laugh from the safety pilot we were fortunate that this understanding safety pilot allowed us to continue our flight tests. Thank you so much safety pilot! We really appreciate the generosity of the safety pilot and we also really appreciate our MuPAL-α! Why? Because the weather conditions were not so good and we sometimes had severe wind gusts during that flight test, but we returned to our base safe and sound even though our C-code had a serious mistake in it! In conclusion, that mistake was not so serious thanks to the safety pilot.

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Author Masayuki Sato on board MuPAL-α for flight test, Japan 2015. (Photo: JAXA – Photograph was taken through bubble window by Masayuki Sato’s colleague.)

As this experience illustrates, mistakes are sometimes made in research work. But we learn, grow, and gather new information when these things happen. Our MuPAL-α is still a wonderful working unique research airplane with its experimental FBW system. Today, we have an international research collaboration working together under an EU-Japan funding, and so far, we have been conducting flight tests without silly mistakes!

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From left to right: Evaluation pilot, ONERA researcher, safety pilot, and author stand in front of MuPAL-α for an EU-Japan joint research mission under the Horizon 2020 Research and Innovation program at the hanger of JAXA Chofu Aerospace Center Aerodrome Branch, Tokyo, Japan, in March 2017. (Photo: JAXA)

What is an IFS?

Excerpt taken from Norman C. Weingarten’s essay on the “HISTORY OF IN-FLIGHT SIMULATION & FLYING QUALITIES RESEARCH AT CALSPAN” in 2003.

“The variable stability airplane was conceived as a device that would permit variation, in flight, of the characteristics or flying qualities of an airplane so a pilot could determine the suitability of these characteristics in actual flight. Today the concept of the variable stability airplane has progressed into true in-flight simulators (IFS) which are routinely used as an extension of ground-based simulators to the flight environment and its real-world cues. These applications include aircraft development, research of flying qualities, systems test, and special pilot training. In-flight simulators use some of the same technologies that go into ground simulators (modeling, control loaders, cockpit displays, and actuation systems) and add to that, aircraft augmentation technologies. IFS computers drive real responses of the aircraft instead of just displays and limited motion systems, and the outside visuals are the real world instead of computer generated.”

AIAA Paper 2003-5464 AIAA Atmospheric Flight Mechanics Conference, August 2003, Austin, TX AIAA Journal of Aircraft, Vol 42, No 2, March/April 2005

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