In free markets-or so the theory goes-good products rise to the top like cream and bad products are weeded out and tossed aside to wither and die. In the real world, its messier than that, thus we have VHS instead of BetaMax, PCs dominate Macs and the overwhelming majority of airplanes have failure-prone dry vacuum pumps instead of bulletproof wet pumps. What happened here? The precise details
seem to be lost to the sands of time but the short version is this: Dry pumps appeared in the mid-1960s, they were substantially cheaper than wet pumps and by the time owners realized how unreliable dry pumps were, the aircraft manufacturers had established them as the defacto standard. And because the OEMs could get away with that, the two wet pump manufacturers-Pesco and Garwin-got out of the wet pump business and eventually disappeared entirely.
In the interim, owners who have always preferred wet pumps over dry pumps have simply retained them, finding shops to overhaul them or in many cases, just letting them perk merrily along without so much as a friendly swipe from a 9/16th-inch wrench. That sort of thing gives some mechanics the willies, but vacuum is vacuum and owners are understandably reluctant to fix what aint busted. As far as we know, theres only one manufacturer of new wet pumps, the Airwolf Filter Corp., an Ohio-based company best known for add-on aircraft oil filter systems and, lately, wet vacuum pumps. At least one company we know of, Airpower, overhauls Pesco and Garwin pumps and markets these through Aircraft Spruce and Specialty. A third company, M-20 Turbos, made a run at the wet pump market with overhauls and a proposed new wet pump, but the company informed us recently that its no longer in the pump business.
Buy Or Not?
The economic ascendancy of dry pumps complicates the wet-pump purchase decision for most owners. Diehard owners of wet pumps tell us theyre not interested in converting to dry pumps. But having suffered through multiple failures, many owners of dry pumps are contemplating a walk on the wet side. But its not so simple. Any airplanes manufactured after the demise of wet pumps-early to mid-1960s-are usually approved only for dry pumps, so even though a wet design will bolt to the same accessory pad, the installation wouldnt be legal without an STC. For any owners interested in converting, Airwolf has a long approved model list (AML) for virtually everything flying. As attractive as wet pumps are and even though Airwolf makes the conversion process easy, we suspect many owners pass on wet pumps because of the hassle and cost. Its just easier to buy a rebuilt Rapco or a Tempest, bolt it on and go flying. At $1599, the Airwolf pump is nearly six times more expensive than the price-leader Rapco overhauls and more than four times as much as top-of-the-line Tempest dry pumps. Moreover, dedicated air/oil
separators for the wet pump are must-have equipment and if you dont have one installed, plan on another $400. Bottom line: Budget $2200 to $2400 to convert from a dry pump to a wet pump. But realistically, wet pumps appear to be so reliable and durable that most owners may never have to replace or even overhaul one. Airwolf, for example, offers a generous 2000-hour/10-year warranty.
How They Work
Wet pumps have such a superior service reputation over dry pumps that youd assume that theyre radically different in design. In fact, they arent. And a better description of a wet pump may be “oil misted” rather than wet, since they are anything but submerged in oil.
As shown in the photos at left, wet pumps and dry pumps share a fundamental design similarity. Both pump or compress air by spinning blades or vanes inside a sealed bore. Both types also use graphite for the vanes, but at this juncture, the two designs diverge.
A wet pumps vanes are three times thicker than those found in the typical dry pump and theyre composed of a softer graphite. While the dry pumps vanes bear and seal against a polished bore inside the pump, they run dry, relying on powdered graphite from the wear process for lubrication.
Wet pumps, on the other hand, are lubricated by a light misting of oil metered into the pump through accessory case inlets. In addition to lubing the vane-to-bore contact surface, the light oil film also seals better, which improves pumping efficiency. In the Airwolf pump, the vanes seal against a cast iron insert, a material that naturally contains embedded graphite, further reducing friction.
In part, this is why wet pumps are more durable. But theres more to the story. Wet pumps are more robustly made than are dry pumps, which accounts for their higher prices. Note that the Airwolf pump has a heavy, precision machined rotor which rotates on a pair of ball bearings, one sealed and one constanly lubricated by engine oil.
Airwolf sent us two pumps for trials, one of which arrived disassembled so we could examine its innards. We were impressed with the overhaul quality of the machine work on this product. The pump body is machined from a single billet of 6061 aluminum and nicely anodized a striking gold color. The body, end cap and sleeve seem to be similarly well machined for perfect fits.
Airwolfs John Kochy told us the companys pump is a direct descendent of the original Pesco pump, whose plant was in Bedford, Ohio, near Airwolfs Middlefield, Ohio headquarters. He makes no claims about reinventing the wheel.
“Our specialty is not inventing something from scratch, “Kochy told us, “but making something better.” Those improvements include the billet body rather the original cast aluminum, plus a handful of minor proprietary changes Kochy declines to reveal. He explains that the largest challenge in building a wet pump-which drives cost upward-is the requirement to hold exceptionally tight mechanical tolerances. For example, all of the clearances in the pump are in the 0.0001 to 0.0003 inch range. “It would be a lot easier if we could open those tolerances up, but if we do, we lose performance.” (This leads us to wonder how efficient overhauled pumps are compared to Airwolfs new models, but the company doing the overhauls didnt respond to our queries.)
From our previous test of pumps, we know that all of the dry pumps and Sigma-Teks new Aeon piston pump deliver adequate vacuum through the entire RPM range, although their output falls off at idle speed. (SeeAviation Consumer April 2007 for the full report.) We wondered how the wet pump would compare.
The Airwolf clearly moves more air than the dry pumps do, probably due to the oily seal between the vane bearing surfaces and the inner bore of the pump. As the lowest possible idle our pump motor could deliver-about 600 RPM-and lower than the typical engine would be idled, the pump was easily delivering 3 CFM at over 5 inches of mercury. At these low RPMs, dry vane pumps typically deliver marginal vacuum, although the Aeon pump did better. We werent surprised that the Airwolf pump was working harder to deliver more vacuum. Our DC test motor drew 9.9 amps when running the dry pump, but nearly 25 amps when the Airwolf pump was running. Apart from high cost, one glaring weakness of wet pumps is their reputation for spewing oil, which finds its way onto the belly of the airplane. The OEMs used this as a selling point in the conversion from wet to dry pumps. And while its true wet pumps are messier than dry pumps, we think the argument is overstated. In running the wet pump on our test stand, we placed a sheet of clean white paper in front of the exhaust vent and collected a barely detectable misting of oil. The exhaust smells warm and oily, but its hardly spewing oil. (Admitedly, it might exhaust more if the pump were being oiled by engine oil.) Airwolfs Kochy told us the pump consumes less than 3 ccs of oil every 10 minutes, which equates to 0.02 quart per hour. That may vary with installation-some owners do complain about an excessively oily belly-but a properly installed air/oil separator catches most of the oil. Just know this: A small number of owners still report serious oil issues.
We have no reservation in saying that a wet vacuum pump is right for every airplane-its just better technology than the dry pump. But its not right for every owner, chiefly because of cost. We think an owner who flies a Cessna 172 or a Piper 180 50 hours a year and never ventures into IMC is better off retaining a dry pump and replacing it as necessary with an overhauled Rapco pump, the value leader in dry pumps.
But for hardcore IFR pilots or those who worry about dry pump failures-and its a question of when, not if, a dry pump will fail-a wet pump is as close to a lifetime component as any of us are likely to get. Backing up instruments is always a risk tradeoff. The belt-and -suspender approach is costly and not always necessary for owners on a budget. We think wet pumps are reliable enough to install in lieu of any other kind of backup, such as electric gyros or electrically driven backup vacuum pumps.
For owners interested in larger 400-series pumps, Airwolf is developing these and expects to have them available within a year. No prices have been set, but dont expect them to be cheap. For more information, contact Airwolf at www.airwolf.com or 800-326-1534.