I am looking at purchasing an SR20 but see that it doesnt have a TACH meter, only the HOBBS. When I flew one the HOBBS didnt run on battery like a Cessna does. When does the HOBBS begin to count? Also does it vary speed like a TACH or not?
Thanks in Advance.
Tracy,
Here’s when it runs:
When on, 1 hour = 1 hour, period.
One can apply a factor of about .8 to determine an equivalent tach time. This was established by our illustrious president, and so is called the “Radomsky Factor”.
IOW, if you’re used to doing something every 100 hours on a plane with a tach (like spark plugs), you can accomplish about the same thing by doing it every 125 hours on the hobbs (that’s what I do).
Some have installed an airswitch to control the hobbs to record actual flight time, but…
It’s open to debate whether that time can be used for maintenance. The AMM calls for the installed hobbs to be used. IOW, if the mags have to be inspected/overhauled at 500 hours, the AMM pretty much states that’s hours based on the installed hobbs, NOT some other way of measuring time. In real life, if the mag manufacturer wanted 500 hours in a regular plane, we could do the inspection at 625 and accomplish the same thing - BUT the AMM would seem to preclude that.
Ed,
Do you think there would be any harm in leaving alt1 and alt2 off while you contact ground, taxied to the runup area etc? Turning on the alt1/2 at runup and then off when you taxi off the runway toward your hangar/tie-down?
Rick
In reply to:
Ed,
Do you think there would be any harm in leaving alt1 and alt2 off while you contact ground, taxied to the runup area etc? Turning on the alt1/2 at runup and then off when you taxi off the runway toward your hangar/tie-down?
Rick
By leaving Alt.1&2 off line wouldn’t you drain the battery?
Raj
Rick,
I’m not a battery expert, but I would be concerned that routinely taxiing/running up with alternators off might reduce the service of the battery as it is not designed for deep-discharge cycling. I think this type of battery, designed for starting, is happier when kept charged and can potentially be damaged by long duration high charge rates like you will get if you routinely discharge it substantially before turning on the alternators.
Bat #1 has a capacity of only 10 amp-hours at a 1-hour discharge rate…less at higher discharge rates. After a normal start I normally see a charge rate of nearly 30 amps for a while, and as I recall the normal load on the battery is in the 15-20 amp range.
With a battery in perfect condition, a normal load with alternators off might be expected to fully discharge the battery in 30-40 minutes, not factoring in the substantial drain from engine start. If your taxi and runup takes 10-15 minutes (?) you might be routinely discharging the battery by as much as 40-50%.
Just my 2 cents worth.
Tim
In reply to:
By leaving Alt.1&2 off line wouldn’t you drain the battery?
Raj
There should be plenty of battery, no pitot heat, landing light etc. Awaiting taxi instructions and then taxing to the runup area would be way less than needed and then the alternators certainly recharge the battery once you turn them on. This is just my common sense logic which has been known to not apply to reality hence the question to ed and others who might know.
In reply to:
Rick,
I’m not a battery expert, but I would be concerned that routinely taxiing/running up with alternators off might reduce the service of the battery as it is not designed for deep-discharge cycling. I think this type of battery, designed for starting, is happier when kept charged and can potentially be damaged by long duration high charge rates like you will get if you routinely discharge it substantially before turning on the alternators.
Bat #1 has a capacity of only 10 amp-hours at a 1-hour discharge rate…less at higher discharge rates. After a normal start I normally see a charge rate of nearly 30 amps for a while, and as I recall the normal load on the battery is in the 15-20 amp range.
With a battery in perfect condition, a normal load with alternators off might be expected to fully discharge the battery in 30-40 minutes, not factoring in the substantial drain from engine start. If your taxi and runup takes 10-15 minutes (?) you might be routinely discharging the battery by as much as 40-50%.
Just my 2 cents worth.
Tim
Beyond the battery life issue, I can think of another reason to not do this. After a start off Batt 1 and running avionics off batt 1 and 2 only (no alt) your going to leave the ground with something less than full batteries.
That is your reserve in case of alternator failure. So, in the interest of keeping the hobbs meter accurate, you will unwittingly launch without your max backup reserve in the batteries. Same is true after landing. Your going to store the plane with less than full batteries and when you need max battery output to turn the starter, you have something less than that.
My take is even if it didn’t harm the batteries (I tend to think its not good for them at a minimum), it is just not a prudent way to operate your system. BTW, think about the charge load on the entire system too. The alternators will try like crazy to quickly charge the batteries. That’s a lot more stress on everything to operate them at that load.