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Tech Talk

By Shawn Coyle | June 1, 2005

Happy With What We Have?

A nice thing about being in flight testing is that you get to see lots of new ideas and technology--some very promising, some flights of fancy. Some have immediate potential to improve aviation and some seem to be solutions looking for problems.

In the helicopter world, we don't see a lot of this new technology put into practice. We lag far behind our fixed-wing brethren in the practical application of technology. Why this is so is grist for another discussion.

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In the fixed-wing business jet world, two items that have gone from "a good idea" to installed, operating hardware include synthetic vision and head-up displays with velocity vectors (or flight path markers) and combinations of the two. The path from concept to execution was not easy, but execution was accomplished. It wasn't cheap, but when the airframe costs upwards of $20 million, another half million doesn't seem like much--especially when it helps guarantee the boss will arrive safely, on time.

If you accept that the reason for the new gear is to guarantee a safe landing at destination, then the helicopter equivalent would be to guarantee a safe arrival at a hover to a helipad regardless of wind or weather. What or how it's done is almost immaterial, as long as it can be done safely (which means meeting some standards of reliability and integrity).

The thing that is really irksome is that this has been done several times in several different areas. Seven years ago, I flew such approaches as part of a demonstration program, and I've sat through presentations at professional forums where others talked about their successes. One of them was an IFR approach to hovering 40 ft. above the water alongside a ship that was under way.

(Those who know a lot about GPS should skip the next part, as it's a very rough description.) "Normal" GPS won't work because the height signal is not accurate enough to provide precise enough height AGL.

One system used a real-time differential GPS (DGPS) ground station with a datalink to the helicopter. The DGPS basically says, "I don't care what the satellites say, I know exactly where I am, so I'll correct the satellite data and send the corrections to the helicopter." (The accuracy can be inches). Another system used a combination of DGPS and millimeter-wave radar to provide redundancy, again with a datalink between ground and air. With an accurate height and position signal, we can fly to the hover. But how?

If you've ever tried to fly a decelerating approach in instrument meteorological conditions from 90 kt. to even 60 kt. indicated airspeed (KIAS), it is not easy. When the speeds go lower than 60 KIAS, the task approaches (no pun intended) impossibility for several reasons. First, the airspeed indicator stops indicating anything useful below 40 KIAS, and ground speed has to come from the GPS. Secondly, heading control becomes extremely difficult--there is not enough inherent directional stability or slip ball cues below 60 KIAS to maintain track and heading if you're doing it manually. Finally, being below minimum power speed (in the region of reversed command in fixed-wing terms) means that a sharp crosscheck is needed to maintain speed and height control, let alone track. All the approaches to a hover have been flown using four-axis (pitch, roll, yaw and collective) flight director cues as a minimum and most use a four-axis autopilot.

The effect was nothing short of magic--follow the flight director cues and watch the airspeed needle slink to zero while still several hundred feet up and arrive at a 50-ft. hover over the desired spot. (Or better yet, punch the right buttons at the correct time and watch the system do its thing), When the "end of approach" light comes on, look up and land, even with a 10-kt. tailwind.

Further reflection on the potential made me realize that, just like fixed-wing airplanes with Category 3B capability, guidance right to touchdown was going to be necessary. The reason is basic performance. Single-engine helicopters are not going to fly these approaches. They lack the necessary redundancies and systems, particularly the four-axis automatic flight control system. So, at some point on the approach, most twins are going to be committed to land if an engine fails. That point is well above the 50-ft. agl decision height for nearly any helicopter I know, based on Category A performance.

But be of good cheer--if the system can give guidance to a 50-ft. hover, it certainly can give it to touchdown. And who has ever heard of not being able to see the ground from 50 ft. in even the most dense fog?

But the question remains--why is this not being brought into everyday service?

Perhaps it's a problem of size of the market relative to cost. Perhaps a question of need. Where and when could you use such a system? Getting the patient to the hospital regardless of the weather? Getting the boss to his destination every time?

But I personally think it's because we're happy with what we have--an affliction of the rotary-wing world.

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