While I was working on my commercial time building last year I had to declare an emergency to the approach controller. The story begins the day prior to declaring my emergency. I was taking off from my home airport for a cross country flight when I noticed my tachometer was indicating a few hundred RPMs lower than it typically indicates in climb. Once I reached pattern altitude, I decided to land back at my airport and squawk the issue.
I had done all of my private and instrument training in this airplane. I was very familiar with the performance of the aircraft since I had logged 100 hours in that specific plane. Once I got back to the flight school, I asked for the squawk book and all of the instructors wanted to know what was wrong with the plane. After telling them it was indicating slightly lower than usual in climb, they all started laughing. They mocked me and said planes are "suppose to indicate lower in climb". I was already aware that planes indicate lower in climb. The reason I decided to terminate the flight was because it was the lowest RPM output I had ever seen in climb in that plane.
Fast forward to the next day I was under the impression that an A&P had looked at the plane. After taking off, I didn't notice any discrepancies. I got to my cross country destination and did a touch and go in Lincoln, NE. This is when I lost all RPM indications on my tachometer. I was about 1,000 feet over the ground when I looked over at my tach and noticed it was indicating zero at full power. This was definitely alarming. However, I kept my composure and started my turn back to the airport. I didn't recall hearing a drop in RPM, however, it was my first time wearing a noise canceling headset and I thought maybe my headset is just REALLY good at its job.
I didn't 100% know what was going on. But I did know I wanted to be on the ground. The Lincoln airport had more traffic than usual that day so I decided to let approach and tower know that I was having issues by declaring an emergency. I wanted to get on the ground as fast as possible and not worry about being vectored around other traffic. The engine seemed to be working just fine but I didn't know if I was about to have an engine failure since my tachometer was already indicating one.
I requested the longer runway at Lincoln since I knew I wanted to come in high in case I did lose my engine. I knew I would float by coming in high which is why I requested the longer runway. I ended up landing at Lincoln and after maintenance looked at the plane, they determined it was just a faulty tachometer. This crossed my mind while I was in the air, but I decided to be extra cautious in case it was something more than just a broken tachometer.
The moral of the story is to be alert and always take safety seriously. The day prior I noticed the initial sign of the faulty tachometer and I was laughed at when I wanted someone to go take a look at it. Safety should always be prioritized and never overlooked. I declared an emergency in case something worse happened while trying to divert back to the runway. I am glad I declared the emergency. Approach and tower helped me out by vectoring other aircraft out of my way and making sure I landed safely.