Date: 201912
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Cessna 140
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Training
Flight phase: Taxi
Airspace: Class D ZZZ
ATC / advisory: Ground ZZZ
Reporter organization: Personal
Reporter function: Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Human factors: Training / Qualification
Anomaly: Deviation / Discrepancy - Procedural FAR; Ground Event / Encounter Loss Of Aircraft Control; Ground Event / Encounter Object; Ground Excursion Taxiway
Detector: Person Flight Crew
When detected: Taxi
Result: Aircraft Aircraft Damaged; Flight Crew Regained Aircraft Control; General Maintenance Action
Contributing factors: Aircraft; Human Factors
Primary problem: Human Factors

Narrative: This was an instructional exercise to introduce taxiing in a tailwheel aircraft. There was no intention to fly. After approximately one hour of discussing how ground handling a tailwheel aircraft is different from a tricycle gear aircraft; the pilots boarded the aircraft and the CFI called Ground Control to establish an acceptable location to taxi. Ground Control instructed the aircraft to taxi on the [1; 2 and 3] taxiways and remain south of [4] taxiway (and clear of all runways). While taxiing within this area; the Private Pilot and the CFI transferred control back-and-forth several times as the unique handling characteristics of the tailwheel aircraft were demonstrated and observed. After approximately 15 minutes of taxiing; the Private Pilot attempted to turn left from [taxiway 1] to [taxiway 2]. During this turn; it is estimated that the aircraft was traveling approximately 8mph when the Private Pilot lost control and the aircraft veered sharply to the left. The CFI attempted to correct by exclaiming 'My airplane!' and applying right rudder and right brake. The CFI also attempted to reduce power since the Private Pilot was adding power through the turn. The Private Pilot; however; did not immediately relinquish control and briefly resisted the CFI's efforts. After a brief moment; the Private Pilot relinquished the controls and the CFI was able to prevent a ground loop but the aircraft had already departed the taxiway into the grass. With the engine at idle and with hard braking; the CFI was simply unable to get the aircraft to stop in time before striking the taxiway sign. Unbeknownst to the Private Pilot and the CFI; the registration status of this aircraft had been changed to 'deregistered' the day before the event. The owner of the aircraft had been trying to renew the registration for a number months with the FAA returning the application multiple times. From what I understand; the owner accidentally let the registration lapse [many months earlier]. When he attempted to renew shortly thereafter; the application was rejected since it was now a new registration and not a renewal. This rejection took several weeks. The new application was submitted along with payment. The check used to pay for the registration cleared the bank [a couple months ago]. Knowing that the FAA is 6-8 weeks behind in processing registrations; we continued to look at the FAA website to monitor status. The night before the event; the owner looked at the website to see that the registration status still had not changed. With the cancelled check and the delay in processing; it was believed that the aircraft was properly registered and that some taxiing exercises would not be an issue. It was not until after the mishap that it was brought to our attention that the new application was again rejected and the aircraft status had changed to deregistered.




Synopsis: C140 Instructor reported taxiway excursion resulted in aircraft striking taxiway sign.