When the Airbus A350 took off for the first time just over two years ago, there were two test pilots and four flight test engineers on board. But on the ground in Toulouse, France, another 70 specialists were monitoring the flight in real time via voice, video and datalinks relayed to the airframer’s telemetry center.
“We’re the eyes of the airplane on the ground,” comments telemetry center leader Jean François Meryet. “We’re not on the airplane, but we feel like we are. We have the same information as them, we have the dataflow that permits us to know what’s going on, and we have the flight crew dialog. We really feel like we’re with them.”
The additional experts on the ground are there to help the flight crew carry out the test flight and achieve its specific goals. And as well as answering specific questions and verifying that the test condition has been achieved, or asking for it to be repeated, the occupants of the telemetry room monitor the aircraft’s structure and systems in real time so they can warn the flight crew of any unsafe condition. The benefit for the engineers on the ground is the immediacy of the readings and the fact that they can better understand the context of each test and the effect of factors such as local weather.
THE SETUP IN SEATTLE
The telemetry (TM) room at Boeing Field in Seattle uses hardware identical to that installed on the aircraft to drive the displays and show the data to the engineers. “We want the same look and feel in the TM room as what they would experience if they were on the airplane,” says Tom Smidt, instrumentation and data systems manager at Boeing Test & Evaluation. “When our analysis engineers fly with the airplane, they can look at data in real time. So the TM room is just an extension of the airplane.”
Engineering values are shown on paper output from strip chart recorders as well as computer screens, all driven by the Airborne Data Analysis and Monitor System (ADAMS). “It’s essentially just an Ethernet network of hardware that runs the TM room,” explains Smidt.
Each of the eight tables in the room is allocated to a different discipline such as structures, flight controls or systems. “They each are looking at their particular things during the test,” he says. And there are more people in the TM room than there would ever be on the aircraft: “On a typical flight on the airplane you might have 10 or 12 people. In the TM room you could have 30. We’ll have more people in the TM room than on a flight because we need to view critical data in real time.”
Nobody is there without a good reason, though. “They have a pretty strict policy that if you’re needed for the test you can be in the room; if you’re not needed for the test you can’t be in the room, however senior you are,” Smidt says. And the test director is the sole point of contact with the aircraft: “There may be a lot of activity going on in the room, but there’s only one person that talks to the airplane.”
Boeing does its test flying in the eastern part of Washington state, about 300 miles to the east of Seattle. “Our TM system operates in the S-band region (2.3GHz),” explains Smidt. Data and VHF voice communications from the aircraft used to be received by a ground station at Moses Lake and was relayed by satellite to Seattle but that has changed in the last three years with the installation of new hardware at a company site at Moses Lake. “Now we’re using the internet to bring both the data and the radio channels back.
Only a limited subset of the data generated on board can be sent to the ground, even with a 15Mbps bandwidth datalink. “On some airplanes that might be everything, but on most airplanes that’s maybe a quarter of what’s available,” Smidt says. The data to be downlinked is selectable and can be changed on the ground between test flights: “If they see an area of interest and they want more information, they can change that. Not on the fly, but before the next flight.”
Data is stored initially on solid state drives with a capacity of around 700GB. In the course of a test flight lasting four to six hours, Smidt says, a 787 might fill up a couple of drives, “but on a 737 you wouldn’t fill up one.” In the ground station, an online RAID server system retains the data for about 30 days before transferring it to tapes, which are held in a Hewlett-Packard tape library. Duplicate copies are stored off-site.
“We have a 12-year retention policy, although we are asked to keep some things longer,” says Smidt. “We can recover data from everything that we did within the last 12 years. If you wanted to request something that you did last year, it might take an hour for the system to locate and download that tape.”
The same system that was used for 787 testing is currently being used for flight tests of the US Air Force’s new KC-46 Pegasus tanker. The next program on the schedule is the 737 MAX, expected to fly next year, and for that there will be changes, Smidt explains – a Crystal server will provide the basic computing power and the current paper strip charts will give way to flat panel displays that should improve reliability.
“The strip chart recorders are mechanical devices, so they need
a little more care and feeding,” he comments. They should also show the airframe oscillations induced during flutter testing more immediately, for example: “I think we’re going to see the results more quickly on the screen because the paper has to travel from wherever the print head is. Some of the engineers are attached to the strip charts – but they’ll get over that.” None of the equipment is hard-wired, he adds: “All of this is programmable, so you can have any piece of information on any channel.”
“I think of the TM system as a tool,” Smidt sums up. “It provides an efficient and safe way of testing the airplane before it is deemed airworthy. The alternative would be to do a test and land, and then look at the data, so it certainly beats that.”
Airbus installed the first TM room at its Toulouse, France headquarters in 1987 for flight tests of the A320. A second room was added in 1990 and a third in 2006, while the military transport aircraft flight test center in Seville gained its own TM room in 2004. The three rooms enable the Airbus master telemetry center in Toulouse to follow three different flight tests simultaneously, and are networked with both Seville and annex telemetry rooms installed in 2009 at Hamburg and Bremen in Germany and at Filton in the UK. Current flight test activity is focused on the re-engined A320neo family.
With its soundproofed, matt black walls, the Toulouse center was designed to minimize the stress of work that demands intense concentration over extended periods of time. Instead of windows, each of the TM rooms has large screens showing high-definition video, including live video transmitted from the cockpit and external cameras on the flight test aircraft. The screens are managed via a video-matrix-driven touchscreen from AMX, while a Bouyer public address system relays audio to the telemetry rooms and the corridor outside.
For the specialists monitoring specific aspects of the tests being flown, there are PCs with twin displays showing the output from Airbus software running on a Linux mainframe server via Oracle Secure Global Desktop (SGD). According to Oracle, the SGD network enables Airbus staff and suppliers to access test data from server-hosted environments via a web browser using a standard corporate PC, allowing experts in the various disciplines to provide nearly instant feedback from their off-site locations, rather than having to travel to Toulouse.
Test flights are carried out in six overlapping zones, each roughly circular and approximately 500km in diameter, that extend from Brittany and the Bay of Biscay on France’s west coast, to its Mediterranean coastal waters in the southeast. There is a dedicated antenna on the ground for each area: line-of-sight range from antenna to aircraft is more than 350km at 40,000ft. The antennas are controlled from the telemetry center in Toulouse, and an Ethernet/IP network relays data from the antenna stations at up to 10Mbps.
Originally, the telemetry signals were transmitted in L-band (1-2GHz). In 1992, the medium was changed to S-band (2-4GHz) and last year there was another switch to C-band (4-8GHz) using the 5.091-5.250GHz frequencies allocated by the International Telecommunications Union to airborne mobile telemetry in 2007. The airborne telemetry equipment consists of a data concentrator, a multiplexer to mix the audio, video and data, a transmitter and the antennas.
One objective of the telemetry center is to enhance the safety of test flights by minimizing the number of crew on board the aircraft, while maximizing the number of analysts on the ground. The other is to make the flight test campaign more efficient by providing real-time analysis support from the ground, which enables the contents of the flight to be optimized and the number of flights to be reduced.
Depending on the criticality of the test, there can be anywhere from three to 50 people following a flight test. The highest number ever was 70 for the first flight of the A350 in June 2013. There are 12 flight test groups covering performance, aerodynamics, engine, handling qualities, flight control, braking, electricity, cabin, flutter, load, autopilot and fuel.
When the A340-600 entered flight test in 2001, a data transmission rate of 0.8Mbps limited the maximum data flow to 2,400 parameters. By the time the A380 flew for the first time in April 2005, the bandwidth had increased to 5.3Mbps and the number of accessible parameters had grown to a maximum of 8,000.
Those figures are still small fractions of the number of parameters that can be accessed – 600,000 in the case of the A350, nearly twice as many as on the A380. These figures are a function of an increase in the number of computer tasks on the newer aircraft and a parallel increase in the number of computers.
During each test maneuver, the telemetry team assesses the incoming data and advises whether it has been completed satisfactorily so that the crew can move to the next step in the test flight plan. The on-board flight test engineer has the final say in all decisions.
The central telemetry room continually analyses essential data such as fuel consumption, loads experienced by different parts of the aircraft structure, and the response of key flight control systems. Two graphic recorders can make instant printouts for closer analysis of any part of the data. Requests can also be sent from the computers in the telemetry room to the aircraft’s test flight computers for more information about the performance of specific systems.
The team of specialist analysts study real-time transmissions from the aircraft on generic or ATA chapter-specific screens. Telemetry equipment is pre-configured based on the results of functional integrated benches and simulators (mainly for systems parts), but also on input derived from computations – finite element stress analysis for loads, computational fluid dynamics for aerodynamics, and so on.
The equipment then interprets flight test behavior and is able to send a warning to the aircraft when the situation becomes critical or dangerous. These telemetry tools also propose real-time automatic monitoring, which can warn the specialist in case of an unexpected event.
The telemetry center can also analyze data received from the aircraft in slightly deferred time. Deferred computation allows for a more detailed analysis, which is then compared with previous models and sent to the design office, aircraft program, other crew and test specialists after the flight test to obtain a global overview of the results.
Bernard Fitzsimons is an aviation journalist specializing in air transport business, technology and operations.