In ground school at Northwest Airlink, they taught us to activate the propeller deice whenever an ice accumulation was detected, i.e., on the wing or the wipers (at night you can't always see the wings that well, but the wipers are 14" in front of your nose). Sounds simple enough. In actual practice, it doesn't work too well.
To understand it, you need to know how a prop deices. Simple- centrifugal force. The outer two thirds of the propeller are turning fast enough so that centrifugal force needs no assistance. Ice builds up and presto! It flings off. The inner third though is not turning fast enough for centrifugal force to work. For this area there is an electric boot. When the prop deice is activated, the boot heats up, melting the underside of the ice accumulation. No more adhesion and so presto! It flings off. So in icing, always put the props to max rpm for greater centrifugal force.
This works great on metal propellers. On a composite propeller though, not so great. Apparently the metal blade is heated up to some degree (partially by the boot and partially by friction) and prop ice that melts and runs back on the metal blade just evaporates or runs off. Remember, it only needs to be 33 degrees or better. A composite blade though does not heat up. Herein lies the problem. When the boot heats up, the ice melts. If there is only a small amount of ice accumulation, then the entire amount of ice melts, not just the underside. Centrifugal force then causes the melted ice to run back onto the propeller. Like water drops running down your windshield, it runs down a line and refreezes. This line runs down the length of the propeller at an angle, not straight back. The prop deice is on a timer, so every three minutes it heats up again. If the ice has not accumulated enough, the boot will heat up, the new ice accumulation will melt again and run down the same line the previous melted ice made. This creates small ridges, which totally ruins your thrust. Remember, props are just wings mounted sideways. These little ice ridges act the same as spoilers do on a wing. As the ice ridge builds, it will do so unevenly from blade to blade. This makes the blades unbalanced and so the shaking begins. First it's a low hum that won't go away. Then over time it builds until everything is vibrating slightly. If you're still in the ice then things start to vibrate more. You can't take a sip of coffee without spilling it on your clean white shirt. Oh the humanity! Eventually the oxygen mask, hanging on a hook behind you, starts rattling on the wall. An experienced Saab 340 pilot knows to wait until the mask rattles before turning on the prop deice. As the boots heat up, the accumulation is flung off and all the vibrations cease. Now the key is to wait until the mask rattles again, so you need to turn off the prop deice as soon as things smooth out. Problem is, this is contrary to every airline's Standard Operating Practice in icing conditions. It's also contrary to what Saab recommends in the Flight Manual.
Winter, around 1992. We're flying the DME arc to the localizer back course approach to Hibbing, Minnesota. Yes, it wasn't that long ago people actually flew DME arcs and back course approaches. So there we were (ah, such a classic line), at 3000' in the clouds, in moderate icing conditions. The landing gear is up and the flaps are up, and we have full power set, yet we're only getting 160 knots indicated. We should be doing 250 plus. The plane is vibrating like crazy too. The prop deice is operating and doing a great job of creating run back on the propellers. The only way to get rid of it of course is to fly into air above freezing, and that isn't happening until we get pulled into the hangar tonight, assuming we make it that far. I discuss with the First Officer my doubts about being able to hold MDA with gear and flaps down. He agrees and we figure that something like 650 fpm should give us a 3 degree descent angle. So we'll hold that and at MDA if we don't see the runway we'll just retract everything and go vibrating our way to Duluth. So down we go, using lots of power to hold 130 knots with gear and approach flaps set. Full flaps felt like it would be too much. Nearing MDA we get the runway in sight and land.
Ten years later. I'm working as the Program Manager for the Saab 340 at FlightSafety International. As such, I get to attend the Saab Operators Conference being held in St Petersburg Florida. That was amazing. Let me digress a bit to say that every morning they had a gigantic buffet set up, with every kind of breakfast food you could imagine, and giant, fancy coffee urns filled with Starbucks coffee. Lunch was out on the grass using tables with white tablecloths, like a wedding almost. Dinner was on the beach, under a big tent, where they had a stage set up and some sort of lively entertainment. It was in a word, amazing. Those Swedes can sure party.
Well I've been selected to give a presentation to all the flight departments. I spent weeks preparing for this, and brought my kick ass PowerPoint presentation along with a stack of reference material. At the conference, the typical presentation was just a review of some system or operating practice. Some were interesting but most were pretty dry. After lunch on the appointed day, with my wife taking a break from snorkeling on the beach, they announce "And now a presentation by David Reed, Program Manager for the Saab 340 at FlightSafety International from the United States." All eyes, experienced people from airlines all over the world, turn and look at me. Dear God don't let me fuck this up. I take a deep breath and launch into my discussion on why the current Saab endorsed propeller deice procedure is wrong. I give a few nervous glances to the Saab representatives but they
haven't pulled out the blow darts, yet. When I was done I got a few questions and thank God, nobody stood up and called me a complete idiot. Actually, it was kinda controversial and challenged the current convention, and to my surprise I got a LOT of people coming up to me later and saying things like "I'd never thought of that. You know, you really hit on something there," or "I smiled when you mentioned the oxygen mask. That's exactly what I do!" Not to toot my own horn too much, but it's really neat when you give a presentation to a large group of highly qualified people and get kudos afterward. I remember my boss, Mike King, told me "I thought those Saab guys were gonna hang you, but when you're right, you're right and I think they agreed with you." That was it, my fifteen minutes of fame. Now, where are those chocolate-creme filled Swedish croissants?