Date: 201910
Local time of day: 1201-1800
Flight conditions: VMC
Weather elements / visibility: 20
Light: Daylight
Aircraft: King Air C90 E90
Operator: Corporate
Operating under: Part 91
Flight plan: None
Mission: Agriculture
Flight phase: Cruise
Airspace: Class C ZZZ
ATC / advisory: TRACON ZZZ
Reporter organization: Government
Reporter function: Captain; Single Pilot; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Human factors: Distraction; Workload; Situational Awareness
Anomaly: Airspace Violation All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Exited Penetrated Airspace; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Environment - Non Weather Related; Human Factors; Procedure; Weather
Primary problem: Human Factors

Narrative: I was flying doing low level work south of ZZZ at approximately 800 ft. AGL. After finishing several hours of this flying the in the region assigned; I was ferrying under VFR back to ZZZ1; where we had been staging out of for the past five days. While passing south of ZZZ; I made it a point to remain below 2;200 ft. MSL; to stay below the 2;600 ft. floor of ZZZ Class C airspace. I was trying to remain as high as possible due to low level turbulence; without getting too close to the lower deck of ZZZ Class C space. We had seen quite a few birds during the course of our flight. While scanning a bit more than usual for birds; I was also tuning in the frequency for ZZZ1 Unicom and AWOS; in preparation for performing my approach checklist. While doing this; I glanced at my altimeter and realized that I had strayed up to 2;450 ft. MSL. I immediately corrected back to 2;200 ft. When approaching ZZZ1 and setting in the current altimeter setting; I realized that I had been flying around 280 ft. higher than my indicated altitude. This would have put me about 8-9 miles south of ZZZ at 2;730 ft. and into the ZZZ Class C.Contributing factors: - This was my last day of two weeks of flying this contract. Distracted by thinking about getting home. - Was doing most of my flying for almost two weeks; using my Radar Altimeter as my primary altitude instrument. This; due to the nature of the low level application work I was contracted to do. I had experienced a near miss with a big bird (hawk or vulture) earlier in the morning in the same area. Was keeping too much emphasis on looking outside and not on appropriately flying the plane. Inappropriate prioritizing. I thought since I was only going to be under the Class C for less than 5 minutes; I could easily hold an altitude for that long; so I decided to stay a bit closer to the Class C than I normally would. Complacency. I was dealing with increasing low-level turbulence on the flight - Had two [passengers] on board and was trying to minimize their exposure to the low level; moderate turbulence. In reality; an extra 500 ft. lower would not have been much different than the 2;200 ft. I decided to fly. Things to different moving forward. LESSONS LEARNED: Call the local approach facility for flight following when flying near protected airspace. I had spoken to ZZZ Approach several times in the past week of flying and they were always friendly and helpful. I should have taken the extra one or two minutes to establish contact with them. Give myself a greater margin around and under protected airspace. Remain vigilant to keep a current Altimeter setting; even when prioritizing the use of the Radar Altimeter. Be more willing to use an autopilot when one is available. Remind myself that I become more vulnerable to mistakes if or when I start to 'let my guard down'; at the end of a long day.




Synopsis: BE90 Captain reported an altimeter error resulted in an airspace incursion.