Wow, it's getting mighty deep in here so give me a moment to get my boots on.
1. It is infinitessemally rare that an aircraft will go blank in a normal cruise configuration. It is much more likely that you'll be in a turn or doing some sort of manuever. Failsafe will NOT right an aircraft and if you are in a turn or upside down, Failsafe will do nothing to save the aircraft and it will crash into wherever or whomever it's pointed at at the time. I get a real kick out of people dreaming up all sorts of scenarios where PCM will "save the day", but in the real world, those scenarios don't exist and if they could, it's a one in a zillion chance that they would actually happen.
("Yea, like you could be flying and the airplane could be pointed RIGHT AT A GROUP OF CHILDREN and, and, and you could lose radio signal and the PCM Failsafe could take over, and, and, and, like, it could, like, save lot's of lives!. It happened, man, it really happened!")
2. You say that programming Failsafe on a PPM RX is nothing more than a mistake looking for a place to happen. How's that different from PCM Failsafe? In both cases, you're depending on the Receiver to "take over" (yea, right) and "do something". The single best failsafe feature you can program is to cut the throttle. But that still doesn't save the aircraft. And let's make up ANOTHER BOGUS scenario... How about you lose signal, the the throttle cuts and the airplane glides right into a GROUP OF CHILDREN and kills a couple of 'em? When if the throttle had stayed at full, the aircraft WOULD have continued on and crashed on the other side of the runway? (Hey, this making up stuff is FUN!!)
3. The claim that PCM is "better" than PPM FM has been rehashed over and over and people trot out "technical" reasons for it's superiority. But in the real world of flying site management and actual flying experience, no one has yet to point out any difference that can be positively assigned to the mode of transmission (people keep touting their anecdotal "evidence" but there are always other variables that easily account for one mode seeming to be supposedly better than the other).
4. We're back to Potato/Potahto. I've flown LOT'S of PCM stuff; it's all I had for several years. And I've flown LOT'S of PPM stuff; it's all I have now. Potato/Potahto.
Highflight
ORIGINAL: tadawson
A good set of failsafes put the aircraft into a glide at low throttle, and as such, if the lockout was due to engine or vibration noise, this will cause you to instantly get it back, and you can dead-stick it in. With PPM, you may never get the throttle to come down, and it aircraft will be doing random violent moves, unlike the PCM system. Even if it comes down, the PCM system will be much more predictable and avoidable for those on the ground. Should you choose to use something like the FMA copilot, then you truly CAN setup PCM failsafe to recover pretty much any attitude.
And in any case, PCM signals have inherently better signal to noise ratios and detection capabilities, even when trasmitted via FM. The closest simple example I can give is voice vs. morse code. Both can travel on the same amplitude modulated carrier, but the code signal will still be readable when the voice can't even be recognized as voice, much less understood. It is a lot easier for the receiver to detect a level transition with a checksum, and pure analog data with no error detection, as in PPM.
And, on the PPM failsafes, I can't help but think (as others already have) that having to program the failsafe on the RX is nothing more than a mistake looking for a place to happen if you move gear around and such.
- Tim