Friends - Over the last couple of days, I took three flights. My Foreflight logbook recorded almost an hour more time than the plane's tach. (4.7 hours v. 3.8 hours). Can anyone explain why there is such a significant difference?
That actually sounds about right. Your times above are perfectly plausible for tach vs. actual. I know that some people will log tach * 1.2 or 1.3 (not a practice I recommend, since there's nothing the FAA says that you can use to defend that practice should you be called out for it). Your 3.8 hours * 1.236 would give you 4.7...
I also have noted that most flights, I have an average of 0.3 hours of loggable non-flight time. 0.3 hours * 3 flights is exactly the 0.9 difference you're seeing.
Depending on what kind of tach you have, the tach will act in one of these ways:
1) Mechanical tach: Generally counts as a percentage of max or cruise RPM - Let's say 2400 RPM. If you're flying along at 2400 RPM for 6 minutes, you'll get 0.1 hours on the tach, while if you're on the ground idling at 800 RPM, you'll need to do that for 18 minutes before you rack up 0.1.
2) Electronic tach: Generally counts 1:1 above a certain RPM, and not at all below. IIRC, EI's threshold is 1300 RPM while Horizon's is 800. Any time spent above that RPM will give you 0.1 for every six minutes, while any time below that RPM will count as 0.
3) Garmin "tach": Generally, the various Garmin gear starts counting once you hit 30 knots. If it's only a GPS that'll be groundspeed, whereas I believe that an EFIS such as the G1000 will use airspeed.
I'm not sure of ForeFlight's exact algorithm since it's been out for a long time, but IIRC when they detect a takeoff, they'll go back to when you were at rest and count from then. So, for example, it'll go back through your taxi time to when you were on the ramp or at the hangars and at rest, and mark that as the start of the recording. Next time you go flying, have someone watch the timer on the track log recording button on your Maps page - Once it starts recording, it'll instantly have 10-15 minutes on it (the time you spent taxiing). Since the FAA's definition of flight time starts "when the aircraft first moves under its own power for the purpose of flight" that's what ForeFlight is capturing.
I have noticed that their end-of-flight auto-stop is not quite as good, especially if I'm in a hurry - If I taxi up, jump out the door, run into the hangar to grab the towbar, push the plane in quickly, run to the car and leave the airport (ie, I never really stop moving at all) sometimes it'll capture my entire drive home. So, to ensure it hasn't done this, go to More -> Track Logs and choose one of the flights in question. Look at the altitude/speed graphs, zoom in on your destination airport, and move the little slider to that point and hit play if you so choose. If you see yourself taxiing up to the ramp and the recording stops, you're all good. If you see yourself walking around, going to your car, etc then take a look at when you first pulled up to your parking spot, note the time on the slider at the bottom and put that in as the ending time on your logbook entry.
The Foreflight tracking is trash. Sometimes it won't log a flight at all, other times it records a 200 hr flight with me driving all over San Diego with it in my trunk
It has improved significantly over time, FWIW.
An elapsed timer is ALWAYS a worthwhile member of the panel.
And it's already part of many panels, whether you know it or not. Most Garmin transponders have both flight and elapsed time functions, that's what I use myself - But even a lot of the older King avionics have an elapsed time function, if you just RTFM to find it.
