Advanced simulation has become a vital adjunct to flight test safety

Faced with unique, difficult and sometimes perilous targets, the integrated test teams for the Bell Boeing V-22 Osprey tiltrotor and Lockheed Martin/Boeing F/A-22 Raptor programmes have been at the forefront of proving the value of advanced simulation techniques in saving time, cost and even lives in flight tests.

"It has become literally indispensable to the programme," says Tom MacDonald, chief test pilot for the MV-22 integrated test team at the US Naval Air Warfare Center Aircraft Division (NAWCAD), Patuxent River, Maryland. For the V-22, troubled by delays, accidents and uncertainty, every minute of simulation in the laboratory at NAWCAD, as well as at Bell and Boeing sites, has helped play a part in saving the tiltrotor, which "was about as extinct a programme as you could have got about four years ago", says MacDonald.

From this nadir, the V-22 has been revived to the point where final preparations are under way for the start of operational evaluation (Opeval) by VMX-22 at MCAS New River, North Carolina between January and May 2005. The tests, involving up to 12 MV-22s, will continue to be supported by the CV-22 Integrated Test Team at Edwards AFB, California. This team, working up to include four CV-22s by early 2005, is focused on completing tests of shipboard suitability, formation flying, austere landings, air-to-air refuelling using a new retractable probe and mission software validation.

The latest phase of flight tests began in 2002 and re-examined the vortex ring-state phenomenon behind a fatal V-22 crash in May 2000, as well as further work on shipboard landings, low-speed hovering, combat manoeuvrability and formation flying.

Subsequent flight tests included potentially hazardous single- and dual-engine failure procedures, which became heavily reliant on a build-up approach using simulators. "Using simulators doesn't prevent the awful things from happening that you read about, but it can prevent a range of bad things and rule out other events that would happen - as well as help you work out how to deal with them," says MacDonald, who adds that "simulation and flight test are symbiotic".

"We did total engine-out auto-rotations in the simulator, and the simulator said that helicopter auto-rotations would not be a pretty thing. In fact, it showed us coming down like a cinder block. The simulator showed it would be more of a stunt than a predictable manoeuvre and, basically, steered us in other directions in terms of survivability tactics," says MacDonald, who explains that, as a result, the recommended method to recover is to tilt the nacelles down and attain the best glide speed available, then flare to a survivable landing. "We demonstrated it by shutting down both engines at 9,000ft [2,750m], then circled dead-stick and brought both engines back. Then we do the rest in the simulator."

The team used a similarly cautious "build-up" method to tackle the vortex ring state encountered during high-rate descent testing. "It's very complex and, mathematically, the model [used in the simulator] isn't that capable of reproducing it. However, we could approach right up to the boundary and that gave us the ability to be able to practise the tricky timing and flying procedures right up to the vortex ring state."

Despite the benefits, particularly to the V-22 programme, MacDonald warns against over-reliance on simulation. "We have to be careful. There is a tendency for programme management to oversell the labs and simulators. The simulator is incredibly important, but it's the not the be-all and end-all. Sometimes you just have to flight test."

Another complex, high-performance programme that benefits from simulation is the Lockheed Martin/Boeing F/A-22, now entering the final month of its dedicated initial operational test and evaluation phase at Edwards AFB. James Brown, lead Lockheed Martin test pilot at the F/A-22 combined test force (CTF), says the use of simulation, and particularly the flight sciences simulator at Edwards, has "saved us millions and millions of dollars in flight test because so many of our test points are very expensive".

The CTF recently completed a complex series of AIM-120 AMRAAM missile shots in various conditions. "I had to perform a missile shot at 40,000ft, Mach 1.6 and 7g. On the surface that doesn't look too big a deal, but it takes a load of gas, transfer time and tankers. If we were to develop this technique in-flight, it would have taken days and multiple sorties," says Brown, who adds: "This flight sciences simulator has been invaluable."

The resulting technique involves entering a split-S manoeuvre with full aft stick and full afterburner at 53,000ft and M1.98 "and that's just to get to the start of the test condition. All of the conditions merge at 40,000ft and the mission is completed at 18,000ft," says Brown.

 "Back in the 1950s we were killing a test pilot a week and I attribute the increase in safety to the improving fidelity of simulation and the way it has been used to ferret out the dangerous places to allow us to approach them appropriately," he says.



Source: Flight International