Page 13 - Volume 13 Number 2

The backside of the propeller bulkhead with an arrow point- ing to the slip rings. This example shows two, which means the propellers have single-segment boots. Props with dual- segment boots have three slip rings.
four-blade prop with four good boots should show 16-20 amps. If a four-blade prop only draws 14 amps, it has one boot that is not heating. If the gauge reads 9 or 10, then two of the four boots are not heating on that side.
Yes, the manual says 14-18 is OK for a four-blade prop, but that’s in theory. In practice, if the gauge reads 14 and ice is slamming against the nose, something is wrong. There’s a bad lead somewhere, or there’s another issue.
Part of the conflict here is that some of the newest boots draw fewer amps, in the 3-4 range. Perhaps the manual tries to compensate for this. In my opinion, a low cycle should be investigated and not brushed off as “within limits.”
Time Will Tell
A single segment system will heat all the boots on one prop for a period (or cycle) of 60-90 seconds; then the prop heat timer switches the electrical current to the other prop for a 60-90-second cycle. As the system switches back and forth from one side to the other, the needle will flicker very slightly, but as long as all the boots on each prop are heating, the amp gauge reading will remain steady.
A dual segment system works in a similar manner but will have four cycles instead of just two: L/H prop inner
FEBRUARY 2019
segments, L/H outer segments, R/H prop inner and R/H outer (not necessarily in that order). If each cycle can be as long as 90 seconds, then it could take up to six minutes to cycle through all the boot segments one time. This is very important to know when troubleshooting prop deice because a cursory glance at the gauge now and then will not reveal the problem. You will recall the pilot mentioned he kept a steady eye on his prop amp gauge throughout several cycles.
Zero Cycles and Brush Blocks
A zero cycle means no boots on the prop are heating. It is highly unlikely that all the prop leads would fail simultaneously. But if the brush block assembly is worn down to the point where it fails to make contact with the slip ring, that will give you a zero cycle every time.
Brush blocks have springs that push the brushes outward, keeping them in constant contact with the slip rings. When the spring becomes fully extended there is not enough pressure to keep that contact.
Brush Blocks and Prop Overhauls
Saving my customers money was always the uppermost in my mind, so when their props came back from overhaul, if the brush blocks looked reasonably good, I would not change them. Over time, however, I noticed prop heat failures occurring within months or maybe
a year of the prop overhaul.
During a propeller overhaul, the slip rings are cleaned up and surfaced so that the brush blocks make optimum contact. This, in turn, makes the brushes wear down faster as they sit with the newly surfaced slip rings.
A lower than usual amp reading on the prop amp gauge could specify one of the prop leads is bad.
KING AIR MAGAZINE • 11