More Than Red Lights Are Needed For Warning

On the night of 16 October a helicopter ambulance with four persons aboard – one an infant patient being evacuated – struck guy-wires reinforcing a radio tower near Aurora, IL. The wires ripped through the helicopter and caused it to plummet to the ground. All four persons were killed in the shattered heap of wreckage.

The National Transportation Safety Board (NTSB) is investigating. One of the big questions is how the helicopter came to be flying so close to the radio tower, which had flashing red lights on the mast to warn of its presence.

But the red anti-collision beacons, many of which are mounted on antenna masts for years, have limitations. Flashing red lights are not easily discriminated as to range at night, let alone relative height. News reports indicate that the red lights on the mast in Aurora were working at the time of the accident, so clearly they are not sufficient.

Even during the day, the threat of hitting masts and guy-wires is very real for low flying aircraft and helicopters. Masts are hard to see. When flying into a rising or setting sun, masts are almost impossible to see ahead.

Masts, in short, are an all-day, every day threat to helicopters. That is especially true at night, even for helicopters in which the pilot is equipped with night vision goggles.

Accident data indicate that 50 U.S. registered helicopters were involved in wire strikes from 1996-2000. That works out to an average rate of ten per year, but the number of dangerous guy-wire impacts – as opposed to electrical wire strikes – is not known.

Unfortunately, equipping helicopters with cutters that snag and cut high-tension electrical wires is probably not a solution. Guy-wires that are surrounding and supporting antennae masts are of a different sort – high-tensile stranded steel, not electrically conductive filaments surrounded by insulation. No rotor blade is going to survive an encounter with a guy-wire or electrical high-tension wire. No cabin or rotor-mast mounted cable cutter is ever going to neatly sever a supporting guy-wire.

The answer is to make the helicopter pilot increasingly aware of the proximate presence of a guy-wire, and to increase such awareness the closer he gets.

One idea is to mount a laser light near the top of each guy-wire; the light would point down the length of the wire to the ground. By this means, some visibility of the wire is provided without creating a nuisance to the public of light pollution.

How well this would illuminate guy-wires for pilots remains unknown. This approach also does nothing to alert pilots during daylight.

Another method involves a simple transponder mounted on the antenna mast and coding an aural message on a discrete frequency. That frequency could be installed in an interrogator aboard the helicopter and triggered by the mast’s transponder at a usefully short range. “Usefully short” infers that nuisance warnings to helicopters passing the mast tangentially, and at a safe distance, would not be triggered (courtesy of the Doppler shift).

A typical message could be a warbler, followed by a “heads up” in the pilot’s headphones along the following lines:

“Caution: a 300 foot tall mast surrounded by guy-wires strung at 45 degrees is on your present track at a distance of 3 nautical miles. You must immediately climb to 350 feet or divert at least 30 degrees left or right of your present track to avoid collision.”

This is not a technically challenging task and, if adopted for both military and civilian use, cost would be nominal.

Enhancements, such as the triggering of a centrally-mounted laser-light show illuminating guy-wire mounted reflectors, could be justified for particularly dangerous masts astride well-traversed routes. The reflectors would make all the difference to a laser light shining down the length of wire.

It should be noted that isolated tall buildings would also benefit from transponders (Chicago’s Sears Tower, Kuala Lumpur’s Petronas Towers, and various other high value edifices).

A higher form and format of warning is required and justified for a higher degree of safety. The NTSB has called for Terrain Awareness and Warning Systems (TAWS) on medical helicopters. So far, the Federal Aviation Administration has not acted on this two and a half year old recommendation. TAWS is entirely self-contained on the helicopter; it compares the helicopter’s position in three-dimensional airspace against a digitized terrain data base in computer memory. The digitized terrain base can contain made-made objects, providing the pilot with a warning of threatening ground or man-made objects ahead. This technology has been mandated for all airliners, and it has prevented a number of crashes.

However, if TAWS is not going to be mandated for helicopters – especially for medical evacuation helicopters, as urged by the NTSB – the tower/building mounted transponder seems an eminently viable and low-cost means of warning pilots that they are flying toward a man-made hazard to navigation.