Random Musings / Bulletins

4/12/2023

'VDO'/'Hobbs' timers usually account for the total time that the engine is running

• the switch in DMP is activated by oil pressure once the engine is running 

• in some aircraft the battery switch is wired to the VDO/Hobbs timer

• the terms 'VDO' and 'Hobbs' only refer to the (near ubiquitous) two manufactures of engine timers

Tach Time

• measures engine revolutions but it's calibrated such that a tenth of an hour of tach time is clicked off when the engine is at cruise RPM for 6 minutes. In other words, if the plane is at cruise RPM, the tach clock will be clicking off tenths of an hour at the same rate as the Hobbs clock, and the same as the watch on your wrist. But if the engine is idling at an RPM speed that's half of what cruise RPM is, then the tach clock will be running at half the speed of the Hobbs clock.

• So, for the tach clock, less "time" is still clocked when the plane is idling on the ramp, or flying at low RPM. For short flights (where ramp idling time is a significant percentage of total time), and flights where you're doing a lot of pattern work (and thus operating at low RPM), tach time will be significantly less than Hobbs time.

Air switch

• counts total time airspeed is activated once a predetermined speed is reached during the take-off roll eg 30kts

Other meters may be referenced at times including;

• gear switch - sensed when the landing gear is pulled up 

• squat switch - sensed when the wheels leave the runway and relieve pressure on the landing gear

As for deciding between what to use, the CAR allows for owner discretion;

- I have paraphrased it as follows;

"Flight time can be measured by any means available, provided it is accurate. A wristwatch can be used, however many aircraft have a mechanical means of measuring TIS and this function may be used, if fitted. The flight time can also be measured by the engine tachometer however, when this method is used, the time recorded will be greater than that indicated by a wristwatch or other mechanical flight time measurement device such as an air switch or a switch that is activated when the aircraft is no longer supported by the undercarriage. if operating at an aerodrome where long taxi and holding periods are involved, the operator should consider using a method that records actual flight time."

• Most schools will maintain aircraft on VDO time AND charge for VDO time to the training pilot - this is the case as a lot of time can be spent 'on the ground' doing run-ups and other such things; plus the aircrafts get knocked around a bit from heavy landings etc 

• Most sole owners will maintain aircraft on Air switch time (as the club had been doing) - this is so as to get the maximum time out of running the aircraft between maintenance cycles

• One could argue that using the tacho would help keep track of the engine doing 'work' and hence the wear on the engine; so it is sage to keep logging on the tacho - especially important for engines that are 'on condition'; there was a bulletin put out by Jabiru once that specifically referenced this idea and therefore it has become a popular among many users. If anyone wonders what the material difference is - it’s really quite small change. 

As much as the US spelling of tyre irks me there is this mentioned in the user manual.