M-20 J (201) / Allegro
Flight Crew Private; Flight Crew Instrument
Flight Crew Last 90 Days 53.3; Flight Crew Total 464.8; Flight Crew Type 388.2
Human-Machine Interface
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Regained Aircraft Control; General Maintenance Action
Aircraft; Human Factors
I was flying on an IFR flight plan to VRB in a Mooney M20J with a King KFC 150 Flight Control System. Conditions were VFR at the airport and enroute; with a scattered cloud layer between approximately 2;000 FT and 4;000 FT MSL (cloud layer thickness of approx. 2;000 FT). Cruise altitude was 6;000 FT. This 'possible pilot deviation' event occurred during the initial approach to VRB. The following is a summary of the events to the best of my recollection.I executed a climb; cruise and initial descent with the flight director/autopilot assisting my workload. These phases of flight were normal. Enroute I was cleared direct to VRB. Approximately; 20-25 miles from TRV; on a heading of 350 degrees; I was instructed to descend to 5;000 FT; then shortly thereafter to 4;000 FT. I had acquired the ATIS at VRB and I requested the VOR 11R Approach. I was instructed to fly to and intercept the 7 DME TRV arc. Approximately 10-15 DME; I asked if I could descend. ATC advised that he needed me to stay at 4;000 FT due to other IFR traffic in the area and keep my speed up (which at that time was approximately 140 KTS).  He assigned a new heading of 330 degrees to intercept the 7 DME arc and maintain 4;000 FT. ATC then advised a heading away from the 7 DME arc of 270 and asked that I slow to 80 KTS. At approximately 8-9 DME; on the approx. 230 radial; ATC advised a heading of 020 and descent to 3;000 FT. Approaching BUEYE ATC advised a descent to 2;000 FT and a left 360 at BUYEYE Intersection (IF) and then cleared for the approach. All this time; ATC was actively communicating with other aircraft. It was during this time; on the initial approach; in and out of cumulus clouds that the plane began porpoiseing numerous times. I began assessing the situation. The AI was giving erroneous information causing the autopilot to abruptly oscillate and porpoise. I disconnected the autopilot and began to hand fly and troubleshoot; now with a slowly failing and ultimately totally inoperable; tumbled AI. The standby vacuum did not improve the AI. I regained control of the aircraft using secondary instruments. I was in and out of the clouds; however not in total IMC conditions. My focus was on flying the plane and assessing the situation. Sometime during troubleshooting at 3;000 FT; ATC queried my altitude. I had lost altitude and was climbing through 2;800 FT at the time. On the approach; before being handed off to the Tower; Miami center gave me a telephone number and directed me to call regarding 'possible pilot deviation.' I called the Center upon landing and spoke to a Supervisor who requested my name; address and pilot certificate number. I was informed that I had a possible pilot deviation of assigned altitude. This occurred while being vectored to the initial approach; penetrating scattered cumulus clouds and preparing for the approach. I was anticipating that ATC would be shortly advising descent to the minimum published approach altitude which would put me below the cloud layer and in visual contact with the horizon and airport. I flew back VFR along the shoreline. Upon reviewing the event; my training to aviate; navigate and communicate was instinctual. I never felt overwhelmed or not in control. However in hindsight; I should have notified ATC that I had an instrument problem even before I concluded it was a failing AI. Also; I initially believed that the standby vacuum would resolve the issue. Reflecting back; I feel there are a number of factors that affected my decision to not notify ATC of the failing/inoperative AI. Things happened quickly. I was not initially positive that the AI had failed. I initially believed that the standby vacuum would resolve the issue. I was within a minute or so of intercepting the final approach course; descending through the scattered cloud layer to VFR conditions. Conditions were not total IMC. My training has afforded me a level of comfort flying partial panel. I didn't feel the necessity to declarean emergency and/or alarm ATC with a false alarm. ATC was actively communicating with other aircraft. I believe now; that this situation could have been avoided by communicating to ATC my status. ATC could have advised a descent to VFR more quickly and I could have canceled IFR and maintained my own separation. In the future; I will communicate with ATC; any; and all potential instrument and/or aircraft abnormalities that could jeopardize my ability to maintain assigned altitudes or headings as soon as practically possible.
M20 experiences an attitude indicator failure during approach through broken clouds but does not inform ATC; resulting in deviations that ATC finds unacceptable.