Date: 201809
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: PA-28 Cherokee/Archer/Dakota/Pillan/Warrior
Operator: FBO
Operating under: Part 91
Flight plan: None
Mission: Training
Flight phase: Cruise
Airspace: Class E ZZZ; Class G ZZZ
Aircraft component: DC Generation
Component problem: Failed
Reporter organization: FBO
Reporter function: Instructor
Reporter qualification: Flight Crew Commercial; Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor
Human factors: Situational Awareness
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Incursion Runway
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Diverted
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: I am a CFI and was flying with a private pilot who requested some flight instruction. We departed to the northwest to do some airwork. The Piper Archer we were flying had had a 100-hour inspection just a few days earlier; and everything on the aircraft seemed to be functioning normally. Less than an hour into the flight; we observed the oil pressure gauge in the yellow arc (below normal pressure) and slowly falling. We decided to immediately proceed to the nearest airport; ZZZ. We were discussing our emergency landing options; in the event of total engine failure; when electrical components began to fail and the ammeter showed a reading of '1.'ZZZ is a non-towered field; so I broadcast our position and intention to land on runway XX on the UNICOM/CTAF frequency. Someone responded that we were transmitting on 'guard' and advised us to switch to the UNICOM frequency. (The comm radio had defaulted to 121.5 as we lost electrical power.) I advised I was unable to change frequencies and would continue to transmit on the emergency frequency. I then heard that person say to someone else; apparently on the field and on that frequency; that we were an emergency aircraft inbound. As we neared the airport; I could see vehicles parked either side of the approach end of runway XX. Then a yellow 'X' on the runway came into view; and I realized the runway/airport was closed. I checked the oil pressure gauge again and; seeing it was continuing to drop and now near zero; chose to continue with the plan to land on runway XX. I made another call; hoping we were still transmitting; to repeat our intention to land on runway XX; I also said we saw the vehicles near the runway and would attempt to land long; so as to stay well clear of them. The vehicles cleared the area and; since the engine seemed to be operating normally all the way to touchdown; we were able to land about halfway down the [long] runway.After landing and shutting down the airplane; two mechanics joined us to help assess our problem. We removed the cowling and saw the alternator belt was shredded - obviously we had lost the alternator and; subsequently; battery power. What I didn't know until then was there is an electronic sender that transmits oil pressure to the gauge; so without electrical power; the gauge failed. Being powered solely by the battery; which was itself being depleted; the oil pressure gauge had failed slowly.Even knowing that; however; I still believe I would have assumed the worst was possible (total engine failure) and executed a landing at the nearest airport. In this case; that necessitated landing on a closed runway.




Synopsis: PA-28 flight instructor reported that the aircraft engine oil pressure indication was dropping and electrical components were failing.