Page 12 - Volume 13 Number 2

Prop leads endure significant wear and tear including constant tension when the engine is running, so they are often the culprit of lower than normal amp readings.
Unfortunately, it is easy to miss a bad lead. The turn- by-hand test described earlier (which is done at every Phase Inspection) doesn’t begin to approximate the conditions in which prop leads operate. Prop leads in questionable condition often pass this test because they are not stretched or subjected to any tension. But as soon as the aircraft is cowled up and taken out for a final ground run, the amp gauge shows a low cycle.
Back when I had my shop, if a customer brought me a King Air and squawked a low cycle on the prop heat (or if I found it myself on the initial ground run) I wouldn’t even bother with the hand-turning test. Instead, I had my guys tug on each prop lead until they found the one that drops the amperage on the gauge. Or better yet, if my avionics guy was available, he’d ohm out each lead.
My first piece of advice to the pilot who called with the left-hand (L/H) prop slinging ice was to convince the shop to take a good look at the prop leads by stretching them out and watching for a low cycle on the amp gauge.
Boot Systems: Single versus Dual Segment
Prop boots on older King Airs may have two segments – an inner segment and an outer segment – and each heats independently. Newer King Airs have single segment boots, whereas the entire length of the boot heats up. In general, three-blade props have the dual segment system and four-blade props have single segments boots, but there are always exceptions to this rule.
The difference is the number of slip rings. Three slip rings mean a dual segment system. Two slip rings means single segment boots. Look at the back side of the prop bulkhead. There is a gap between the bulkhead and the cowling through which you can see the brass-colored slip rings.
Do you have a dual segment system? If you’re not sure, find out. The dual segment system requires that you act with greater anticipation of possible icing conditions and get your prop heat fired up well in advance. The alternating between inboard and outboard segments, delivers less consistent heating than the single segment system.
Again, I repeat, prop heat must be deployed in advance rather than after the fact. It may be called a deice system, but it functions as an anti-ice system.
Prop Amp Gauge
Whether you have a single or dual segment system, if a boot or segment is not heating as it should, you’ve got a low cycle. Ice will accumulate, fly off and slam into the fuselage. To check for a low cycle, keep your eyes glued to your prop amp gauge.
All King Airs have the same gauge regardless of single or dual segments. If the boots are operating normally, each boot (or boot segment) draws around 4-5 amps.
So, let’s do the math. A three-blade prop with three good boots should show 12-15 amps on the gauge. A
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10 • KING AIR MAGAZINE
FEBRUARY 2019