HS 125 Series (Hawker 800) flight crew reported CDU #2 began to display random and nonsensical characters into the scratch pad while on the LEGS page during cruise. The flight crew continued to destination airport and made a precautionary landing.
Synopsis
HS 125 Series (Hawker 800) flight crew reported CDU #2 began to display random and nonsensical characters into the scratch pad while on the LEGS page during cruise. The flight crew continued to destination airport and made a precautionary landing.
Narrative
While enroute after departure; our CDU #2 (Proline 21 version of the Hawker 800xp) began to input random and non-sensical characters into the scratch pad while on the LEGS page. It would then attempt to execute the action without any crew input. We intervened and cancelled the inputs. As a crew; we made the decision to keep that unit only on our COMM (radio) page to avoid any additional attempts by the CDU to input erroneous navigational commands. During the flight; we also noticed that the radio frequency would occasionally become quiet which was unusual for daytime traffic on a Center frequency. This forced us to conduct radio checks a couple of times and once verify a frequency on guard when we received no reply. We did not notice anything unusual with the comms page at this stage of the flight but thought it was odd as we were both actively listening to the radio and had guard frequency up on our Comm 2 channel.While on vectors for a visual approach at DAL; we received and acknowledged the request to descend to 3;000 ft. and heading of 160 for the planned visual approach to 13L. We then also acknowledged the Controller asking us if we had a visual on a 737 4 miles in front of us; expecting the Controller to clear us behind the traffic to land. We then never heard another transmission. At this time; my Copilot then put Tower frequency in standby in anticipation of an additional turn to final and then a hand off to Tower. When it appeared that we were approaching runway center line without additional instructions; I queried the Controller. The box had switched the frequency to Tower without command. The Tower Controller then stated that he tried to contact us (with no reply) but then stated just maintain our heading and remain at 3;000 ft.; which we did. On subsequent vectors around the field; we were transferred to approach again for vectors to final. On the subsequent left downwind for the field; we were both hyper-vigilant of a potential comms malfunction. When the downwind became elongated towards DFW airspace; I queried the Controller. The box once again switched from approach to Tower without any input by myself or the Copilot. I made the query (realizing it was with Tower after a few seconds) just as I heard a call out with our call sign on guard. The Tower Controller then asked if I had the field in sight; which I did and stated I did. He then cleared us to turn towards the field and cleared us for the visual on 13R which we acknowledged. While descending on short final; CDU #2 then flickered to all yellow text indicating that comms were offline for a few seconds before it returned to normal. We landed without further incident and did not notice any additional issues with our radio while taxiing to parking. No deviation or further contact was requested by ATC after landing. However; it was clear that our control unit was malfunctioning. Once at the FBO; we contacted our Maintenance Personnel to discuss the issue further. We made the decision as a crew that continuing the trip back to home station was not possible with a malfunctioning CDU without additional maintenance actions. We wrote up the malfunctioning CDU in our forms and coordinated with Maintenance for a replacement of the malfunctioning unit before any additional flight should be made in the aircraft.
Second reporter narrative
intervened and cancelled the inputs. As a crew; we made the decision to keep that unit only on our COMM (radio) page to avoid any additional attempts by the CDU to input erroneous navigational commands. While enroute; we also noticed that the radio frequency would occasionally become quiet which was unusual for daytime traffic on a Center frequency. This forced us to conduct radio checks a couple of times while enroute and once verify a frequency on guard when we received no reply. We did not notice anything unusual with the comms page at this stage of the flight but thought it was unusual as we were both actively listening to the radio and had guard frequency up on our Comm 2 channel.While on vectors for a visual approach at DAL; we received and acknowledged the request to descend to 3;000 ft. and heading of 160 for the planned visual approach to 13L. We then also acknowledged the Controller asking us if we had a visual on a 737 4 miles in front of us; expecting the Controller to clear us behind the traffic to land. We then never heard another transmission. At this time; I (Copilot) then put Tower frequency in standby in anticipation of an additional turn to final and then a hand off to Tower. When it appeared that we were approaching runway center line without additional instructions; the Captain queried the Controller. The box had switched the frequency to Tower without command. The Tower Controller then stated that he tried to contact us (with no reply) but then stated just maintain our heading and remain at 3000 ft.; which we did. On subsequent vectors around the field; we were transferred to approach again for vectors to final. On the subsequent left downwind for the field; we were both hyper-vigilant of a potential comms malfunction. When the downwind became elongated towards DFW airspace; my Captain queried the Controller. The box once again switched from approach to Tower without any input by myself or the Captain. The Captain made the query (realizing it was with Tower after a few seconds) just as I heard a call out with our call sign on guard. The Controller then asked if I had the field in sight; which I did and stated I did. He then cleared us to turn towards the field and cleared us for the visual on 13R which we acknowledged. While descending on short final; CDU #2 then flickered to all yellow text indicating that comms were offline for a few seconds before it returned to normal. We landed without further incident and did not notice any additional issues with our radio while taxiing to parking. No deviation or further contact was requested by ATC after landing. However; it was clear that our control unit was malfunctioning. Once at the FBO; we contacted our Maintenance personnel to discuss the issue further. We made the decision as a crew that continuing the trip back to home station was not possible with a malfunctioning CDU without additional maintenance actions. We wrote up the malfunctioning CDU in our forms and coordinated with Maintenance for a replacement of the malfunctioning unit before any flight should be made in the aircraft.
NASA callback
Reporter stated the root cause was found by Maintenance to be a defective CDU. Unit was replaced and the problem did not recur.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.