Posted: Sat Jan 28, 2006 11:02 am
I've mentioned elsewhere that 170's exist with 20,000 hours on them in hard, low-level useage. For some reason generally the public seems to hit on 5,000 hours as nearing the end of usefulness for light planes, but I don't understand why. The rule-of-thumb I was taught was to look for very low time airplanes, or very high time airplanes. The low time airplanes have less wear, and the high time airframes have had their worn parts replaced. (Lot's of DC-3's and Boeings have 100's of thousands of hours on them and are still safely airborne.)
The recording tach is primarily used to record engine time. The tach is most accurate at the cruising rpm range of the associated engine. An engine being operated at faster rpm will record slightly more time, while the engine operated at less than rated cruising rpm will record slightly less time. The difference is only slight. (After all, if the tach is designed to indicate revolutions per minute.... then by necessity it'd have a fairly accurate idea of what a minute is.) This is consistent with what might be expected as regards engine wear. An engine that is lightly operated will recieve less wear then one that is run at full rpm all the time, and the tach will reflect that. I consider it a more accurate reflection of engine wear than the type of hour meter which only records time ON or OFF, regardless of operating regime or conditions.
Using the recording tachometer to keep track of airframe hours isn't as far off as one might imagine either. An airplane taxying around on the ground at low rpm wouldn't be recording a whole lot of flight time.... and that's good since it's not flying. The record will be corrected somewhat by the extra few minutes recorded during takeoff, climb, and high-power operations. (None of which does anything to keep a record of the more important, in my opinion, number of cycles. After all, a low wing airplane's wings are still supporting the weight of the aircraft while it's parked, and while the high wing airplane's wings aren't, they do undergo a certain amount of additonal flexing cycle. The number of cycles on flaps, flight controls/bearings, engine controls, ...even the number/direction of full-motion throttle movements within each flight is considered important by some engineers,...the number of takeoffs/landings, brake applications, etc. etc. are useful in predicting overhaul/service requirements,... in most cases maybe even more useful than actual flight time.)
It's the total picture of chronological vs flight times, cycles, service/repair history, storage conditions (outdoor? indoor? dry hangar? preserved? coastal? desert?) , and actual condition, ... which are what really determine an airplane's value. IMHO.
The recording tach is primarily used to record engine time. The tach is most accurate at the cruising rpm range of the associated engine. An engine being operated at faster rpm will record slightly more time, while the engine operated at less than rated cruising rpm will record slightly less time. The difference is only slight. (After all, if the tach is designed to indicate revolutions per minute.... then by necessity it'd have a fairly accurate idea of what a minute is.) This is consistent with what might be expected as regards engine wear. An engine that is lightly operated will recieve less wear then one that is run at full rpm all the time, and the tach will reflect that. I consider it a more accurate reflection of engine wear than the type of hour meter which only records time ON or OFF, regardless of operating regime or conditions.
Using the recording tachometer to keep track of airframe hours isn't as far off as one might imagine either. An airplane taxying around on the ground at low rpm wouldn't be recording a whole lot of flight time.... and that's good since it's not flying. The record will be corrected somewhat by the extra few minutes recorded during takeoff, climb, and high-power operations. (None of which does anything to keep a record of the more important, in my opinion, number of cycles. After all, a low wing airplane's wings are still supporting the weight of the aircraft while it's parked, and while the high wing airplane's wings aren't, they do undergo a certain amount of additonal flexing cycle. The number of cycles on flaps, flight controls/bearings, engine controls, ...even the number/direction of full-motion throttle movements within each flight is considered important by some engineers,...the number of takeoffs/landings, brake applications, etc. etc. are useful in predicting overhaul/service requirements,... in most cases maybe even more useful than actual flight time.)
It's the total picture of chronological vs flight times, cycles, service/repair history, storage conditions (outdoor? indoor? dry hangar? preserved? coastal? desert?) , and actual condition, ... which are what really determine an airplane's value. IMHO.
