CID Approach Controller and aircraft First Officer reported aircraft made an incorrect turn and descent while on vector for RNAV approach resulted in CFTT event.
Synopsis
CID Approach Controller and aircraft First Officer reported aircraft made an incorrect turn and descent while on vector for RNAV approach resulted in CFTT event.
Narrative
Aircraft X was at 3300 feet MSL when I issued a vector of 280 degrees and instructed the aircraft to maintain 3300 until established on the final approach course for the RNAV approach to Runway 25 at IOW. The pilot read back the approach clearance and then stated they had a PIREP for me when I was ready. I observed the aircraft to slightly start turning for the vector issued so I told the pilot to go ahead with the PIREP. While the pilot was reading the PIREP to me; I observed the aircraft turning back to its original track direct to BUCKA; an initial approach fix on the that approach procedure. After the aircraft was through the approach course and the pilot unkeyed the radio; I noticed the aircraft was descending to an unsafe altitude and I instructed the pilot to climb and maintain 3000 feet. Then I immediately issued a low altitude alert and advised the pilot that the minimum vectoring altitude in their area was 3200 feet. The pilot acknowledged and climbed to 3200 feet at which point I issued a vector of 230 degrees to join the approach but also to vector them away from a known obstruction in case of an additional unprecedented descent in that area. The pilot then joined the final approach course and asked if they were still cleared for the approach. I responded in the affirmative and restated an approach clearance for clarity. The pilot was then shortly after approved to switch to the common traffic advisory frequency and landed safely at IOW airport. The pilot cancelled their IFR flight plan on the RCO (Remote Communications Outlet) clearance delivery frequency. Shortly after I issued a BRASHER" warning for a possible pilot deviation and advised the pilot contact me via phone. I recommend the RNAV approach procedure for Runway 25 at IOW airport be changed to remove the HEWRI waypoint as this waypoint has a published crossing altitude of 2800 feet; which is below the minimum vectoring altitude in that area. The pilot had contacted us after landing via phone and advised that they as confused their altitude with the 280 vector that was assigned and the altitude at HEWRI of 2800 feet. The pilot also stated they had to disengage the auto pilot as the aircraft began to descend unprecedentedly."
Second reporter narrative
We were assigned direct BUCKA but informed we would be given vectors for the RNAV 25 into IOW while in a broken layer. We descended down to 3300 as assigned prior to BUCKA and direct to BUCKA was a course/heading of around 350. About 2 miles prior to BUCKA we were assigned heading 280 to intercept the final approach course; maintaining 3300 until established. After acknowledging with ATC I began giving a PIREP of a cloud layer above us we had passed though on our descent. The pilot flying set 280 and heading mode to intercept as the autopilot was engaged. I stopped giving the PIREP as I noticed our the airplane began a slight bank to the right before continuing to the left and the nose began to drop unexpectedly. The pilot flying took manual control of the aircraft and increased our pitch to a positive rate while continuing a left turn to intercept the final approach course as ATC reported an altitude alert. I didn't see the lowest altitude we reached; but I think somewhere around 2800 ft. I reported climbing through 3000 ft with ATC. I called for the pilot flying to climb back to 3300 and he wanted to make sure we were still cleared for the approach. I queried ATC and they stated we were cleared for the approach; so I didn't continue to advocate for 3300 as we were established inbound inside of BUCKA at this point. We continued without incident from there to land on Runway 25 at IOW fully breaking out of the broken layer around 2300 MSL. I am not sure what caused the issue as we approached BUCKA. This airplane in our fleet with Collins Proline avionics; I am not sure if that was a factor as the other aircraft have dual Universals. Sometimes the autopilot can be a bit finicky and begin to roll to the left or right opposite the direction we intend for a second; so I didn't initially think much of the slight bank to the right. However; it could have been possible that we had selected direct to the procedure turn outbound BUCKA or hold at BUCKA in the Proline. I don't think this is the case as if so it auto-sequenced to HEWRI as the next point by the time I looked at our course after verifying we were climbing. I am not sure what caused the nose to drop with the associated altitude loss either. If the autopilot was trying to reach an altitude at BUCKA with us being so close to it; a high rate of change would likely be needed; but no altitude was set at BUCKA in the proline and we had previously leveled off at 3300 and were in ALT (Altitude) mode prior to being cleared for the approach. I believe VNAV was selected by the PF (Pilot Flying) to intercept the glidepath; but even if a different mode had been accidentally selected in the moment; I can't think of anything that would have caused us to enter a sharp drop in pitch purely from a change of mode selection. Another potential factor could have been the tight turn that we were left with when ATC cleared us for the approach. I don't know if the airplane 'decided' a steeper turn was needed to make the intercept and the nose drop may have had something to do with a potentially high load factor if altitude had been maintained. A more proactive approach with ATC could have also better set us up to intercept the final approach course. Additionally; if more vigilance were maintained as the airplane intercepted the final approach course; maybe something indicating advance notice of the autopilot actions or observations to better identify the issue could have been made. Giving a PIREP is something that could have waited or been accomplished sooner in order to be more vigilant in monitoring the approach as we intercept the final course.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.