An HS125-800's altitude capture function failed to capture FL290 during a climb with the autopilot engaged in the Flight Level Change (FLC) mode and 290 in the capture window.
Synopsis
An HS125-800's altitude capture function failed to capture FL290 during a climb with the autopilot engaged in the Flight Level Change (FLC) mode and 290 in the capture window.
Narrative
Climbout to the south then assigned heading to the west with step climb to FL290 to intercept the departure routing. Climb to FL290 was a step climb without the need to level off. As 'pilot not flying' I set the assigned altitude in the altitude alerter and both crew members confirmed and acknowledged the set altitude. 1000 FT before reaching assigned altitude both crew members call out '1000 FT to go.' Out of FL280; approaching FL290; I noticed that the autopilot did not reduce the rate of climb while approaching FL290. I alerted the pilot flying; 'altitude...altitude...' Pilot flying immediately reduced power and disengaged the autopilot in the process and leveled off at FL290. Then he reengaged the autopilot and continued with the flight. A little while later; someone mentioned over the radio that they had a TA on their TCAS. The transmission was garbled and we checked our TCAS not knowing who it was and where it might be. Our TCAS traffic display was all BLUE; with NO RA; NO TA; NO aural warning and NO visual instructions on any screen. (TCAS had checked OKAY during preflight). ATC said; 'Maintain FL290 and contact next Controller.' I responded with 'We are level at FL290 and nothing/all blue on TCAS.' Problem: Collins Pro-Line 21 has a KNOWN problem for not catching the selected preset altitude under certain conditions. Having selected FLC is one part of the problem when set in conjunction with another selection of the system such as a new altitude in the altitude selector. I do not believe they know the precise combination and or cannot duplicate it with confidence to develop a fix. This is the second time this has happened to me; once as a pilot flying and now as pilot not flying. This is definitely twice too often! This flaw does not only put my license in danger but can obviously create unintended and unanticipated consequences. The problem is when it happens everybody scatters and it is all blamed on the flight crew. With the way the Pro-Line 21 functions; I can NOT trust it and when I fly I can not relax at all; especially during the phase of the flight when altitude changes happen. The autopilot should help to reduce the workload and make it safer. In this case it has the opposite effect and makes it harder on the crew because you can NOT TRUST IT. The next problem is that you cannot tell anyone because nobody really wants to hear about it and fix it. Problem TCAS: If someone claims they have an indication and I do not (if it actually was us) who is right or wrong. If it is an TA it is not that serious; but what about a possible RA? We are trained to respond to it without thinking. If he has something and I do not; what do I do? Again these small incidences put doubt in your mind. All the experts on the ground can claim what they want and study what they want; however; I am faced with a decision and very little time to do the right thing. ATC should rethink their procedures and off-set airplanes a little more so they do not cross right overhead. Have airplanes level off from their climb or descent 10 or 15 miles before they cross another aircraft's path. Situation: We as a flight crew are obligated to use everything which has been designed and put it in the cockpit. From the start of the flight to the finish we are held responsible that every piece of equipment works as advertised; no exception. Even when equipment malfunctions without our knowledge we are held responsible. Everybody from ATC to the FAA seems to have a way out of their responsibility and put the blame on the flight crew. Collins has a system certified by the FAA which obviously has a known problem. What has been happening: NOTHING!! My first incident was over three years ago and they knew then already that they had a problem. TCAS is a great system. It works almost flawless; but the word is 'almost.' And still when something goes wrong the first blame is on the crew and maybe later on the system when nobody remembers that there was anincident. Human error is prevalent but everybody needs to realize that the black boxes screw up also. They are not the Gods everybody wants them to be every time. We always believe the boxes but not the crew and that is a major mistake in this industry. We lose a lot of valuable input.
Second reporter narrative
On climbout we were being stepped up in 1 or 2 thousand foot increments by Center. I was using the FLC feature of the Pro-Line 21 system installed on this aircraft. There have been reports of problems with the altitude capture when using FLC in this installation. When approaching assigned altitude of FL290; the pilot not flying noticed that altitude capture had not engaged and advised me. We ballooned approximately 200 FT above altitude as I disconnected autopilot and descended back to FL290 and reengaged autopilot and altitude hold. Continued flight without further incident.
NASA callback
The reporter stated that because he personally received a FAA letter of warning while flying this same aircraft and experiencing the same altitude capture failure he is usually very vigilant. Other pilots at his Company have had the same experience with the same aircraft. The other 800XP in their fleet has a different radio combination and does not have this failure to capture flaw. The last time this happened to the reporter the aircraft was in FLC and the altitude set window was changed to a lower setting during the climb but the autopilot did not capture the changed setting. After that event; the reporter's chief pilot found a service bulletin from the manufacturer reporting a design flaw which intermittently prevents the autopilot from capturing the altitude set. The reporter did not have the bulletin with him but recalled that a certain combination of FLC and Altitude Set button presses can trigger the failure to capture.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.