kodoz Posted October 8, 2011 Report Share Posted October 8, 2011 The EP for an electrical fire in flight for the R22 is Master off, Alt off, land immediately. The POH follows this with a warning that the Low Rotor RPM warning system and governor are inoperative. The tachs should continue to run off battery power via a bypass, even with the master switched off, right? I'm not looking for the answer from the POH, I'm more interested in your experience. Quote Link to comment Share on other sites More sharing options...
jimbo2181 Posted October 9, 2011 Report Share Posted October 9, 2011 The tachs are self generating. Basically the phonic wheel creates a small electrical current and the frequency of that current is what is translated into a speed or percentage. Quote Link to comment Share on other sites More sharing options...
Pohi Posted October 9, 2011 Report Share Posted October 9, 2011 Yeah, the tach won't go away. They go through the clutch switch too. This you can do while on the ramp running. I used to do it with students to demonstrate, battery switch off, look at the changes, then back on, alt off/on, then both off. Quote Link to comment Share on other sites More sharing options...
lelebebbel Posted October 9, 2011 Report Share Posted October 9, 2011 (edited) The tachs are self generating. The tachs in the R22 / R44 are NOT self generating. Yes, they will continue to function with the master and alternator off, as long as the clutch switch is in the "engage" position, but they will stop working if the battery goes flat (or their power supply is otherwise disabled). Leaving the helicopter parked with the clutch switch in the "engaged" position over the weekend can cause a flat battery, because the tachs are still powered. Edited October 9, 2011 by lelebebbel Quote Link to comment Share on other sites More sharing options...
kodoz Posted October 9, 2011 Author Report Share Posted October 9, 2011 Yeah, the tach won't go away. They go through the clutch switch too. This you can do while on the ramp running. I used to do it with students to demonstrate, battery switch off, look at the changes, then back on, alt off/on, then both off. That's what I was doing, but the tachs didn't do what they were supposed to do. Color me surprised. Guess I'll be talking to the mechanic... thx for the responses. Quote Link to comment Share on other sites More sharing options...
C of G Posted October 9, 2011 Report Share Posted October 9, 2011 That's what I was doing, but the tachs didn't do what they were supposed to do. Color me surprised. Guess I'll be talking to the mechanic... thx for the responses. This is kinda the funny thing about ep's and system failures: Sometimes it makes you doubt what you know, when in reality the system has a defect. There are plenty of stories about pilots misdiagnosing or worse, rationalizing the sequence of events as they unfold in the cockpit. Always a good idea to ask. Sometimes better to ask more than one person... 2 Quote Link to comment Share on other sites More sharing options...
Pohi Posted October 10, 2011 Report Share Posted October 10, 2011 It might sound silly, but the clutch switch has to be closed for the tachs to pick up (with battery master and alternator switches off). Quote Link to comment Share on other sites More sharing options...
iChris Posted October 10, 2011 Report Share Posted October 10, 2011 (edited) The tachs in the R22 / R44 are NOT self generating. Yes, they will continue to function with the master and alternator off, as long as the clutch switch is in the "engage" position, but they will stop working if the battery goes flat (or their power supply is otherwise disabled). Correct ...... from the R22 Maintenance Manual: The Bell 206 and MD 500 have self-generated dual tachometers, requiring no electrical power. The Bell 407, on the other hand, requires power for its electronic dual tachometer. Your R22, R44, and R66 all need electrical power. Edited October 10, 2011 by iChris 1 Quote Link to comment Share on other sites More sharing options...
iChris Posted October 10, 2011 Report Share Posted October 10, 2011 (edited) That's what I was doing, but the tachs didn't do what they were supposed to do. Color me surprised. Guess I'll be talking to the mechanic... You're saying there's a problem with the system. Not working as advertised. The tachs are supplied 12VDC from the Tach Bus. The Tach Bus receives its power via the D.C. Bus when the battery switch is on and/or the alternator is on-line. However, when the battery switch is off and the alternator is off-line, the Tach Bus is power directly from the battery through an engaged clutch switch. Note the direct wiring from the battery to the clutch switch. Common issues are wiring or connection problems between battery and clutch switch, clutch switch and Tach bus, or bad clutch switch. Those diodes at the Tach Bus are also important. They isolate the two power inputs leading to the Tach Bus. If the diode to the right shorted everything would appear normal until the battery switch was turned off and the alternator goes off-line. Under those conditions along with an engaged clutch switch, the battery is not only supplying power to the Tach Bus but also the D.C. bus (via the shorted diode) and the rest of the electrical system. Edited October 10, 2011 by iChris 1 Quote Link to comment Share on other sites More sharing options...
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