The instrument landing system (ILS) is a really wonderful gadget. When everythings working as it should be, gently keep the aircraft aligned with the transmitted electronic beam, wait long enough and a runway materializes where only a few moments before there was nothing. Pure magic. Its basic technology was perfected long ago, and the fact it remains the primary precision approach is truly remarkable. Of course, the ILS is not without its limitations. For one, it requires substantial site preparation and cannot be installed at just any airport or runway. Its basic characteristics require understanding, too, including the likelihood of false glideslopes, back courses and the possibility of interference from nearby buildings and aircraft. To function correctly, the localizer and glideslope antennas must be placed in specific locations; not all runway environments can accommodate these requirements. What does any of this have to do with accidents? Just this: When an aircraft reaches decision height with a centered glideslope, presuming a steady-state descent by maintaining the current power settings, pitch attitude and consequent rate of descent, the aircraft is almost
Put another way, upon reaching decision height and finding the runway environment in sight, do nothing: The aircraft already is at an altitude and in a position from which a normal landing can be made. Thats the way it was designed, thats the way it works and thats the way it should be flown. Instrument pilots-and especially experienced Learjet pilots operating under Part 135-know this. Why, then, didnt this Learjet crew leave well enough alone for a few more seconds?
Background
On June 2, 2006, at 1440 Eastern time, a Gates Learjet 35A was destroyed when it impacted water and light stanchions while approaching Groton-New London Airport (GON), Groton, Conn. The two airline transport pilots were fatally injured; the three passengers incurred minor injuries. Instrument conditions prevailed for the non-scheduled/on-demand passenger flight conducted under FAR Part 135.
As the airplane approached Groton, it briefly entered a hold before being cleared for the ILS Runway 5 approach. According to the cockpit voice recorder, the crew completed the approach checklist, including a brief of the approach and the missed approach procedures. The crew also obtained the ATIS broadcast, which included two miles visibility and a broken ceiling of 100 feet.
The 18,750-hour captain was the pilot flying. Four seconds before reaching decision height, the first officer said “got some ground contact.” The captain responded, “I got lights” about the same time the first officer stated, “got the, got the lights.” One second later, there was a tone, similar to an autopilot/yaw dampener disconnect. One second after that, the first officer said, “continue,” then, “you still on the ah” and the captain responded with, “whoa.” The recording ended one second later.
A witness fishing from a nearby sandbar could barely see the approach light stanchions in the water because of the fog. He saw the accident airplane was very low before it hit one of the stanchions.
Investigation
The ILS Runway 5 approach is a standard one, with decision height at 207 feet msl and a three-degree glideslope. Weather included winds from 190 degrees true at seven knots, two miles visibility in mist and a broken cloud layer at 100 feet.
Approximately 2000 feet from the runway, a wooden light stanchion was damaged just above the waterline. Another stanchion, about 1600 feet from the runway, was toppled, with the wreckage located next to it. Examination of the wreckage revealed no mechanical anomalies. All flight control surfaces were accounted for, and flight control continuity was confirmed from the ailerons, rudder and elevators to the cockpit. The landing gear was confirmed down and there was no evidence of engine distress.
The engine control units were recovered and their stored data examined. During the last 60 seconds of the accident flight, both engines were operating at a “typical approach power setting.” About six seconds before the end of the data stream, the power levers were commanded back from an approach power setting to flight idle. The power levers reached idle stop about two seconds later and remained there for approximately two seconds before being advanced about one second prior to the end of the data.
Probable Cause
The National Transportation Safety Board determined the accidents probable cause to include, “The crews failure to properly monitor the airplanes altitude, which resulted in the captains inadvertent descent of the airplane into water. Contributing to the accident were the foggy weather conditions, and the captains decision to descend below the decision height without sufficient visual cues.”
Of course, things happen fast in a Lear on an ILS. Even in a perfect world, the time between breaking out at decision height and touchdown would have been very short. The margin for error, similarly, was very narrow, only 200 feet.
Well never know whether the captain consciously descended below the glideslope. We do know he punched off the autopilot in preparation for touchdown and then reduced power. We dont know if the power reduction was accompanied by a slight pitch up to maintain the glideslope, or if he was accepting an increased sink rate and proceeding visually. Even though the approach lights were in sight, we also dont know how much more of the runway environment the crew could see.
We do know, however, the weather pretty much sucked and the crew probably had a real fear of missing the approach. Once they became visual, the captain probably wanted to stay that way, so he strayed below the glideslope.
As the sidebar on the opposite page demonstrates, the ILS glideslope and precision runway markings, working together, lead an approaching aircraft to the runway and provide visual cues of where to touch down. To reach the runway upon breaking out, all we really have to do is nothing. If were on the glideslope and on speed, leave well enough alone. This crew didnt, and presents a lesson for the rest of us.
