2024-08 · NASA ASRS report 2153170
Corporate jet flight crew reported descending below charted altitude on approach to BOI.
I was pilot flying (PF) and we were descending to 10;000 via the Powda One STAR. The approach briefing was to expect a visual approach to runway 12; which we would back up with the ILS-12. The pilot monitoring (PM) checked in with approach control descending to 10;000 via the Powda One. As planned; the PM subsequently requested the ILS-12; KICDO (IAF) transition after the STAR. The response was a casual; 'That's approved; and you can go direct KICDO at this time.' We turned direct KICDO and continued descent to 10;000. In VMC conditions of clear skies and unlimited visibility; out of 11;000 descending to 10;000; we recognized the altitude would be close to a distant peak off the left of the nose which was verified by the PM as 9;650ft. As we decided we needed to go back to 11;000; the approach controller called sounding confused and climbed us to 11;000 with a 20 degree right turn. This was immediately followed up with a turn to 300deg and seconds later a turn to 360deg. After reaching the 360 heading we were descended back to 10;000 before we had even reached 11;000 and within a minute we were given direct KICDO. Due to the flight conditions and our awareness of the surrounding terrain; at no time was the aircraft in danger of a terrain alert. Cause: This event was caused by being cleared direct to a fix off the STAR without an associated altitude clearance. Suggestion: Proactively requesting a new altitude clearance when cleared off a published route which was governing our descent or climb.
We were flying the POWDA ONE RNAV arrival transitioning to the ILS Rwy 12. On the arrival we were given a descend via clearance; selected the lowest altitude of 10000; and selected VNAV. While on the arrival we told the Controller we wanted to fly the ILS from the IAF named KICDO. From KICDO there is an arcing transition to the final approach course. The Controller cleared us direct to KICDO. Since we were pulled off the arrival we were no longer on the descend via portion and still had 10000 in the altitude selector. At this point we should have been at 11000 to adequately clear terrain in front of us. We had leveled at 10000. I looked again at the chart and we agreed we needed to climb to 11000 and started a climb. It should be noted that we were in VMC conditions and had visual confirmation of the terrain. At that moment the Controller also wondered why we were at 10000 ft. I said we agreed with her and were in the climb to 11000. She was happy with that and turned us to the right for a radar vector. After a couple additional heading changes we intercepted the ILS course and made a normal approach and landing.Cause: Staying in a VNAV descent while being pulled off a charted route and not confirming an altitude assignment.Suggestion: When deviating from a charted route with prescribed altitudes at along track fixes; obtain and confirm an altitude assignment from ATC.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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