The ‘Essais en vol’ (flight test) division of the French General Directorate for Armament (DGA), which is the government agency responsible for the program management, development and procurement of weapon systems for the French Armed Forces, is in the process of replacing its existing flying testbed fleet, which currently consists of six rather old Dassault Mystère 20 aircraft. To do so, it has contracted with Sabena Technics to provide a modified Fokker 100 for what the DGA describes as the ABE-NG (Avion Banc d’Essais – Nouvelle Génération) program. In turn, Sabena Technics teamed up with Fokker Services, the Type Certificate (TC) holder of the F100, to help achieve certification with a DOA-to-DOA agreement – Sabena Technics as STC holder and Fokker Services responsible for external changes and their structural consequences.
The aircraft in question, a Fokker 100 (F28 Mk0100) powered by Rolls-Royce Tay 650 engines that had previously served in the Régional airline fleet operating Air France flights, was immediately dubbed ‘Pinocchio’ by the test team because of its peculiar nose.
This was the result of a set of modifications that involved adding a fighter nose pod borrowed from the Rafale fighter, two new underwing hard points with dummy MICA missiles pods, and a recce pod positioned under the fuselage. The work was completed by several structural modifications and reinforcements, internal test benches, and extensive cabling to connect sensors, benches and operator stations. At first glance one could say it’s a Part-25 fighter, but to the test team it was simply ‘Pinocchio’.
The effort required for the DGA-requested EASA certification was impressive: aerodynamic and structural modifications required the STC to be almost a complete recertification of the aircraft – a derivative, as such.
Managing the program was a particularly complex task as it required the coordination of different and distant engineering centers between France and the Netherlands. However, the program is proof of how a strong team integration effort can help shorten a large program.
Work encompassed a feasibility study, preliminary and extensive simulation and wind tunnel testing, the modification and ferrying of the aircraft, and the flight test campaign. All certification subjects were included – performance, handling qualities, stalls, avionics, flutter and, to make things more complicated, many tests had to be repeated according to the aircraft’s various external configurations.
The feasibility study highlighted a challenging situation: very few Part 25 aircraft have been equipped with wing stores and a fighter nose, so the first step was to perform an extensive aerodynamic investigation in the wind tunnel and via CFD simulation. The results provided very useful information and helped determine the position and design of hard points and the nose fairing.
Confident in the results obtained from the preliminary analysis phase, the flight test campaign was designed according to the various aircraft configurations (with or without fighter nose, missiles or recce pod, for example) and resulted in a predicted list of more than 44 successful flights.
The subjects covered by the flight test campaign were: shakedown/instrumentation check; envelope openings; position error correction (PEC); stalls; flutter/aeroelastics; flight handling; avionics; and performance.
Before the actual campaign started, attention was given to team-building activities: three parties were to be involved and later a fourth team from EASA would join. Team-building included two sessions of simulator rehearsal to revise flight test techniques and onboard responsibilities.
Modifications were carried out at the Sabena Technics site in Dinard, France, where ground vibration testing also took place to pave the way for the later flutter tests.
The first flight took off from Dinard, with the aircraft flown to Bordeaux, where the full campaign would take place. Bordeaux was chosen as it is the location of Sabena Technics’ engineering and flight test facilities, as well as for its proximity to the DGA Flight Testing ground station at Cazaux Air Force Base. The Bordeaux site also offered dedicated ATC and telemetry, essential for performing flight handling and flutter tests. Telemetry data was monitored by Fokker specialists at the Bordeaux telemetry room, with support from Cazaux DGA Flight Testing, which covered the entire test area and included an intercom downlink.
Once installed at Bordeaux, the ABE-NG began a tight schedule of tests, with flights always flown with a joint team from Sabena Technics, DGA Flight Testing and Fokker Services. The tests were divided into configuration-related blocks, each with its own flight envelope opening for the four external configurations, from the basic aircraft to all stores installed.
A number of specialists continuously received and verified the data, both in Bordeaux and at Fokker Services HQ in Hoofddorp, the Netherlands. Time-critical data analysis was carried out on a daily basis and the results were fed back to the program management to reconfirm the need for all the tests initially planned. In fact, as a result of this system of negative feedback control, the total number of flights was limited to just 43. Further support came from the local climate – good weather conditions are present almost all year long in Bordeaux and this helped limit the number of missed flight tests due to adverse meteorological conditions.
Flight test provisions
In addition to standard data acquisition sensors, the aircraft was equipped with a number of flight test provisions required to deal with different aspects of the campaign, which were provided by Sabena Technics. One of the first anticipated issues to be solved was the validation of the aircraft angle-of-attack vanes and pitot tubes, as they are positioned in an area where the airflow may have been potentially altered by the installed fighter nose. To cope with this, a swivel-head air-data boom was installed on the left-hand wing tip. It included an air-data computer and provided comparative data for angle of attack, slip, total and static pressure, and total air temperature.
The captain’s control wheel and rudder pedals were equipped with force sensors, while classic tufts on the upper sides of the wings and lateral sides of the pylons were monitored by cameras installed inside the cabin and in the flap track fairings on both sides. A water ballast system helped to change the center of gravity (CG) in flight, avoiding dead ground time.
Finally, flight test displays were installed in the cockpit and at the three flight test engineer positions to enable the test crew to see the parameters not normally displayed in a transport aircraft, but required for correct test execution without ground confirmation.
Stall testing: a key challenge
As well as some common administrative issues, risk mitigation was the main challenge of the program. During the original F100 type certificate flight test campaign, a serious incident occurred when a prototype entered a deep stall while performing stall tests with an open-loop pull technique as required by regulations.
This event resulted in studies and discussions to identify alternatives other than the installation of expensive recovery means such as spin chutes or tail rockets, which, in addition to their considerable expense, would have generated structural and aerodynamic modifications altering the aircraft’s profile. Therefore, during stall tests (but also though the entire campaign), a gradual build-up approach was followed, starting tests with mid-CG and moving toward the envelope boundaries. Additionally, the test sequence went from straight, low entry-rate stalls to turning high-rate ones, always monitoring the angle of attack against calculated and historical values. The maximum aft CG position could be limited due to the specific ABE-NG configuration.
Fortunately the open-loop pull technique is no longer required by regulations, and with all the above mitigations, the stall test campaign could be performed with an acceptable safety level and without need for recovery means – the only real limit being what the human stomach could withstand.
Once the handling qualities tests were completed, the team hosted representatives from EASA to witness a selection of previously performed tests. In fact, EASA flight test personnel were involved from the beginning of the campaign, advising on engineering choices and participating in the preliminary phase, including the simulator rehearsals. This facilitated common and complete understanding of the program and avoided unplanned changes. EASA colleagues also gave useful advice on the correct execution of some tests, such as acceptable stall identification methods.
Four witnessing tests were performed in two phases of the campaign, all giving positive feedback and encouraging the team to continue.
Data analysis and certification
After completion of the campaign, all Fokker Services’ specialists worked hard to complete the data analysis and prepare the reports that were to be used for certification. Fokker Services provided feedback to Sabena Technics in the form of technical reports, consistent with the two-way communication that marked the cooperation between the two DOA companies at all times.
Sabena Technics later submitted and followed up all the information to EASA, which eventually granted the company with the Supplemental Type Certificate in October 2015.
So what key lessons were learned? Strong cooperation based on mutual trust of certification stakeholders makes it possible to reduce the effort of a large modification – even when switching from a transport aircraft to a virtual ‘fighter’. This was achieved without compromising the quality of data collected or increasing the stress levels of the team members involved. Team building and risk mitigation resulted in safe and effective ‘right first time’ test execution. The project was also positive from a human interaction perspective: individuals learned from each other and different cultural backgrounds made for a more interesting experience, especially during time off work.
While perhaps this cannot be labeled as innovative technology, the cooperation between Sabena Technics, Fokker Services and DGA Flight Testing could perhaps feature in test pilot school manuals under a chapter headed ‘Effective teamwork’.
Nunzio Nicola Caccavo is a flight test engineer with the Flight Physics and Operations Support team at Fokker Services