Where is it going now?
Possibly some of the most common five words spoken by pilots as they struggle with their Flight Management Systems (FMS).
The Challenge
Those thoughts must have been on the minds of the crew of a mid-sized turboprop after they loaded the approach into the airport where they intended to drop off the aircraft for maintenance.
They tried the Localizer Performance with Vertical Guidance (LPV) approach three times and each time the FMS failed to arm or activate the approach. This was the only usable approach to the airport that day. As luck would have it, the weather cleared enough permit a visual approach.
The operator contacted the FMS manufacturer who advised that this particular approach design had a feature that made autopilot coupling in some aircraft problematic. Following the OEM’s instructions the FMS had been configured to prevent the approach from being armed or activated.
The operator asked ADS for help. We discovered that at the last revision of approach design standards a new design element had been added. The approach designers could now put a turn in the final approach course at the Final Approach Waypoint (FAWP) of up to 30 degrees. The FMS manufacturer knew that if an aircraft that relied on the classic Approach mode of the autopilot to couple Area Navigation (RNAV) approaches, the aircraft would not manage the turn at the FAWP and a dangerous situation could develop. The approach the operator was trying to use was one of those approaches.
Another business jet operator was struggling with the same difficulty but with a different outcome. Their aircraft was configured to allow these kinds of approaches. This operator’s problem was that while the approach would arm and activate, and initially couple properly, when the turn came up at the FAWP the aircraft not only did not make the turn, but turned in the wrong direction. They had also contacted the FMS manufacturer but had not gotten a complete answer yet.
Other operators reported this too. The answer they always received was to just explain the problem and make sure the configuration on all these aircraft was set to prevent the FMS from arming or activating these approaches.
This prevented them from doing the approach at all. What do you do though if you need to get into that airport? Suppose this was not your destination but your alternate? In fairness, if you studied the approach plates carefully, or took careful note when entering a flight plan, you could see which approaches would not be usable. That didn’t solve anything.
Our approach
ADS dug deeper. By then there were over 30 of these approaches in Canada. Two were LPV and the rest were Lateral Navigation (LNAV) approaches. You could see why the approach designers took advantage of this new allowance. Approaches were bent to avoid airspace, obstacles and terrain. These were all approaches that would not have existed before, or would have had much higher minimum approach limits.
We talked to approach designers and they were all planning to use this feature when necessary, to make approaches work in similar circumstances.
The Solution
Disabling them was not ideal. The pilots could fly them. And the autopilots could track them laterally as long as you didn’t use the Approach mode of the autopilot. For vertical coupling, other means had been traditionally employed to control the descent such as Vertical Speed mode or other vertical modes of the autopilot.
We were convinced the answer was training and using consistent procedures in the flight deck to identify and fly these approaches. We took our ideas to Transport Canada and to the operators. Working as a team we designed and certified standard operating procedures to fly these types of approaches.
The Result
ADS solved the problem. Collaborating with operators and TCCA we developed approved procedures. We also worked with NAV Canada to publish a Canadian Aviation Safety Alert (2018-1)
