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This article begins with a disclaimer: although the title refers to an old dog, its not really fair to call the Bell CH-135 (UH-1N) Twin Huey a dog. It was a solid reliable workhorse, borne of a family of helicopters that transformed both rotary-wing aviation and military operations. Of course, its also the same machine I cut my helicopter teeth on, and its incredibly difficult to say bad things about your first operational helicopter type (I know Ive tried to break lots of student test pilots of that habit, and it dies hard!).
But I digress. The particular old dog Im talking about is also an old friend: N212TP, a former Canadian Forces CH-135 (previous registration 135103) that I first flew in 1977. Thirty-five years later, its been equipped with new toys, and earlier this year I found myself at the National Test Pilot School (NTPS) in Mojave, Calif. getting ready to test them out.
Teaching for the Test
First, some background information on NTPS. This unique school is one of the few places in the world where you can go to fly a whole variety of different aircraft and learn how to evaluate their performance and handling in a structured, careful manner. This is not the same thing as simply learning how to fly. For example, as Ive mentioned previously (see p.116, Vertical, June-July 2012), part of the process involves convincing students that, if they are unable to fly a maneuver accurately, the problem is almost certainly in the machine, rather than themselves. Learning how to look for faults in the machine is not an easy process, but its necessary if one is to evaluate equipment for either military operations or civil certification.
One of the real problems with learning how to evaluate aircraft is that any aircraft that is in operational service is going to be, by definition, pretty suitable (else it wouldnt have made it into operational service!). Thus to get practice in identifying faults, test pilot candidates must be exposed to a wide variety of machines, each one with particular characteristics that make it less than perfect. Consequently, NTPS has a large number of aircraft at its disposal (indeed, between rotary- and fixed-wing, there are nearly 20 different types in its hangars). One of these is N212TP. With good performance and pretty reasonable handling characteristics but also some minor issues the Twin Huey is ideal for test pilot training.
Just as its necessary to have experience across different airframes to be able to identify good and bad handling characteristics, its necessary to have some experience with different avionics systems and displays in order to know whats good or bad. (Its sort of like trying to decide whether the latest version of Microsoft Windows is good: to make an informed assessment, you cant just compare it to an older version; you need to have seen Mac, Linux and Unix systems as well.)
There are several fixed-wing airplanes in the NTPS fleet that have modern glass cockpits, and the helicopter students are given training in those machines but helicopters are different! So it has historically been difficult for helicopter students to make informed comparisons about how these new cockpit displays compare to each other and to the old steam gauges.
One of the few manufacturers of low-cost, helicopter-specific systems for retrofit is Cobham, which has developed an inexpensive automatic flight control system/autopilot and a range of electronic flight instrument systems for helicopters.
Recently, NTPS took advantage of this availability by installing a glass cockpit and an automatic flight control system (AFCS) from Cobham subsidiary Chelton Flight Systems in N212TP. On my recent demo flight at NTPS, I got to see the results.
Taking it for a Spin
The installations in the Twin Huey were pretty straightforward: the attitude indicator and horizontal situation indicator were replaced by electronic displays, and the autopilot controls were added to the center console. Before our flight, Mike Hill, the senior helicopter instructor at NTPS, briefed me on what we would be looking at namely the AFCS modes and the display concepts during typical instrument flight rules (IFR) and visual flight rules (VFR) flights and on normal and emergency procedures. Walking onto the flight line brought a rush of memories back from my own days as an instructor at NTPS: it was a typically hot Mojave day, with a light breeze wafting the warm air a tiny bit.
Although I last flew N212TP in 2004, my history with the type goes back a long way. In fact, I may be the last Canadian Forces (ex or otherwise) operational Twin Huey pilot to still fly the type! The start checks came back in a flash, and I got all the switches and procedures right first time. After a quick review of the electronic displays, we were ready to go.
Lift-off to the hover was with the AFCS on, and the effect it had on handling was immediately and positively obvious. The first thing I noticed was that the stick forces were higher than I remember from the days when I flew with just the force trim on: these new forces were just right (and present all the time, unlike the force trim of yesteryear). Secondly, the aircraft was pretty steady there was no wandering of pitch and roll attitude when the stick was steady. Not that the UH-1 does this a lot, but like most unstabilized helicopters, it does it enough to notice.
The Chelton glass panel displays (attitude and horizontal displays) also incorporate the controller for flight plans and approaches, and to evaluate them, we set out to find a waypoint that would feed us in for a GPS approach. The waypoint desired was about 10 miles away, and we climbed through the warm air to set up for the approach. Not being that up to speed with the system and being distracted by the rush of memories involved with flying this machine in once-familiar surroundings! I didnt enter the flight plan correctly, but we soon got that sorted out. I was impressed by the clarity and detail of the information displayed: airspace restrictions were very evident, and we stayed out of the many nearby military ranges without difficulty.
There was an occasional bit of mild turbulence as we were flying, and the aircraft responded to this as I remembered it doing in the past. What made it different this time was that the AFCS automatically corrected for each transgression about half-a-second before I would have put in an input. Uncanny how it could read my mind!
The navigation part of the display was easy to read and understand, and as we approached the waypoint to start the approach, one of its nicest features became evident. The attitude display featured the almost-ubiquitous highway in the sky symbology, but it also depicted the waypoint as a circle of increasing diameter as we approached it. Normally, waypoints only show up on the horizontal situation display; I was pleasantly surprised to also see them on the attitude display.
We completed the approach, and broke off to deliberately fly toward some high ground to demonstrate the systems terrain warnings. They were compelling and added significantly to situational awareness a nice thing to have when flying in terrain that can bite back.
Back at Mojave Air and Space Port, I set up for an approach to spend some time in the hover. The approach was as smooth as if we were on rails: the ability to set the attitude and not have to make corrections was obvious. Transitioning to the hover took very few cyclic corrections, and then the real beauty of the system showed I could hover for over a minute without touching the controls! Although the helicopter moved around a bit due to the vagaries of the light wind, no cyclic corrections were needed.
(For those who want to know, the norm for this helicopter in similar conditions is a correction every five to eight seconds. We make these corrections instinctively, but we do have to make them, which means that changing radio frequencies or trying to read a map in the hover is a prolonged exercise. Its sort of like trying to use a cellphone while driving in traffic not that you would ever, ever do that!)
Changing position in the hover was likewise very easy. Without using the trim release, I just pushed against the cyclic forces until the desired response happened; once over the new spot, I simply released the cyclic. Turning off the AFCS showed the old helicopter, so a side-by-side comparison of workload was easy which is a definite requirement if youre trying to teach fledgling test pilots and engineers about handling qualities.
Overall, I found the entire Cobham/Chelton package to be an impressive system, and one Id be happy to have in any military light/commercial medium helicopter. The reduction in workload in the hover and cruise was marked, and the displays were a definite improvement over any mechanical system.
As the NTPS installation demonstrates, however, these capabilities arent just nice for day-to-day flying: theyre also being used to educate the next generation of test pilots on what helicopters are capable of. And as test pilots raise the bar for whats good, aircraft get better for all of us.
Shawn Coyle began helicopter life in the Canadian Air Force, and was fortunate enough to attend Empire Test Pilot School. This started a non-stop flight testing career, including teaching at three test pilot schools and working for Transport Canada as an engineering test pilot. Shawn is currently developing helicopter simulators and teaching seminars on helicopter flying.