So why is not premixed Prist sold in Europe? I understand their are health concerns there, but it seems these should be less with premix than using cans.

Here is the story on Prist requirements for PT6 aircraft, at least ones I was familiar with when designing fuel systems:

Part 23 requires prevention of fuel icing and, on some newer models, requires a fuel filter impending bypass indication.

The PT6 Installation Manual requires an airframe fuel filter, upstream of the engine fuel filter and fuel heater.

The only way to meet both these requirements is to require Prist or to add an airframe fuel heater to protect the airframe fuel filter. Nobody does this due to cost, weight, complication, failure modes etc.

So why is not premixed Prist sold in Europe? I understand their are health concerns there, but it seems these should be less with premix than using cans.

Here is the story on Prist requirements for PT6 aircraft, at least ones I was familiar with when designing fuel systems:

Part 23 requires prevention of fuel icing and, on some newer models, requires a fuel filter impending bypass indication.

The PT6 Installation Manual requires an airframe fuel filter, upstream of the engine fuel filter and fuel heater.

The only way to meet both these requirements is to require Prist or to add an airframe fuel heater to protect the airframe fuel filter. Nobody does this due to cost, weight, complication, failure modes etc.

Well, no King Air requires Prist...

Robert

Andreas, I am pretty sure all PT6 engines have a fuel/oil heat exchanger. However at least on the Conquest I, there is an airframe fuel filter before the fuel/oil HX which is not heated. The filter has a bypass in case the element gets clogged and there's an annunciator in the cockpit for that. But the POH still requires PRIST. I am guessing the King Air has a heater fuel filter which gets them out of requiring PRIST except for exceptionally cold OATs. I seem to remember a story from recurrent training about a Conquest that had fuel icing problems due to no PRIST. They descended and the problem resolved but I would not count on that in all circumstances (Butte, MT as you pointed out).

The PC12 has a fuel/oil heat exchanger but is downstream from the fuel filter that’s located closer to the fuel tanks so the PC12 requires anti-icing additive for all flight operations below 0°C.

The issue with the PC12 needing prist is with ice crystals forming in the fuel filter which will pop the buypass valve. There is no indication in flight that the filter is bypassed other than possibly a Fuel Pressure Low CAS or fuel pump/lights that are not displaying normal operation. There are also issues with ice forming in the tanks at the fuel pickup locations as that was suspected to cause the fuel imbalance in the Butte crash.

Many of the FBOs (outside the us) that are not premix have cans of prist in stock and most will add it for you. It’s good SOP to bring your own prist however. Interestingly, the only place I’ve ever had to use the cans of prist in the US is at DFW and they add prist with cans (vs having prist injected on the truck). I’m only guessing so they don’t have a risk of fuel contamination coming from prist on the truck for the airlines.

The set-up of the C425 and the King air C90A / B seems to be pretty much the same. Non heated filter, bypass, annunciator.

However, there must be reasons for the C425 requiring FSII and the C90A not requiring if oil temperature is above the POH limits for OAT and ALT. Maybe the non heated fuel filter of the KA is usually warmer, being at the firewall, or the filter or the bypass are larger.

There is simply no Prist Premix outside North America: not in Europe, and AFAIK neither in Asia nor in Africa. The big iron don’t need it. Well, in Africa, they might tell you it‘s in, but it is not.

Having to use cans of Prist in most cases and not being able to have my FBO at my airport filling up my aircraft without me being present (and the guy being busy with a Hawker when I arrive, me having a departure slot) is very close to a KO criteria for me. I like performance, economy, cabin and cockpit of the C425.

The fuel filter on the 425 in just behind the engine mounted to the firewall. I can't see it being all that different from the KA assuming the KA is not heated. I still would like to know what PC12 operators are doing in Europe...

The fuel icing requirement was added by amendment 23-15 in 1974. At least some King Air models had this requirement. I looked up the 200 TCDS and it was there.

Even with this requirement, there is possible room for interpretration as to whether both the airframe and engine fuel filter need to be protected. One theory could be that it is all right for the airframe filter to bypass as long as the engine fuel filter doesn't. This becomes less practical if a bypass indication is required and/or provided, due to difficulty in specifying whether bypass is normal or not. I have no knowledge of King Air means of compliance.

The 425 airframe fuel filter is not on the firewall, it is in the wing root. The hydraulic filter is on the firewall.

I stand corrected. I was mixing those up. It's in the stub wing right? In that little well next to the hopper tank along with the fuel selector valve?

That could explain the difference.
Thank you, Bill!

No prist needed in the Merlin. Very handy if traveling outside the US.

Yes, that is where it is.

Please could you send me the conquest spreadsheet as mentioned in this thread. My internet mail is jwillemse1011@gmail.com

Pretty sure I remember hearing that the Meridian does not.

Something to consider, re comparing cabin usability between the Conquest 1 and Cheyenne:

The Cheyenne is essentially a pressurised Navajo so the cabin door is aft of the club seating, with a potty seat facing the door. You go up the steps & turn left between the two forward facing seats to enter the club seating area. To the right of the door is the aft baggage area.

The Conquest 1 is the same as the C414/421 cabin: The door is between the two LH side facing seats. Most people position the LH fwd facing seat on its rails so that it is behind the door. That's to make space for getting in through the door. However when that's done then the aft most seat becomes unusable. The other option is to slide the LH fwd facing club seat so that it is ahead of the door - but then there is little to no leg room between those two facing seats. The potty seat is tiny and awkward to use. It faces sideways into the LH fwd facing club seat in the behind-the-door position - with only a few inches of leg room. There isn't really an aft baggage area in the Cessna, apart from a tiny space behind the aft seat.

All in all, I find the PA31 series cabin to be much more usable than the C414/421/425 cabin.

The giant nose of the C414/421/425 is wonderful. More cockpit room in the Cessna too
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