Monday, August 27, 2007

Wet VAC Pump Versus a Dry Pump

An airplane is made of a series of systems that come together to form a reliable, redundant and safe mode of transportation. There are, however, some systems that don't seem to get much thought until they fail. The aircraft vacuum system is one of these areas. It is one of the least thought about systems, yet is one that will kill you in a second if you are not on your game in Instrument Meteorological Conditions.

Flying in instrument conditions, which is commonly called IFR (Instrument Flight Rules) or IMC (Instrument Meteorological Conditions), is when one is navigating an aircraft without outside visual reference of the ground or horizon. Flying IFR requires that the pilot is able to visualize where they are at all times while simultaneously maintaining control of the craft by sole reference of the on board instruments.

Most modern airplanes (non-PFD fitted aircraft) have what is commonly referred to as the T (sometimes called Basic-T) configuration. This is referring to the setup of pilot's instrument panel. The standard T Configuration was developed by the military so that a pilot could transition from plane to plane and instantly be comfortable with primary flight instruments. This standard carried forward to most aircraft built after 1953.

The top of the T contains the airspeed indicator (Airspeed), the attitude indicator (AI) and the altimeter (ALT). The bottom of the T consists of the Directional Gyro (DG) that is sometimes called the Heading Indicator. The secondary instruments are generally placed to the left and right of the DG where the Turn Coordinator (TC) is on the left side and the Vertical Speed Indicator (VSI) is on the right.

System Redundancy
Flight instruments are designed for one to be a primary and the others to cross-check and validate that the primary is accurate. For example, the Attitude Indicator displays a virtual horizon and a stationary line that looks like an airplane will show if you are in a climb, decent or are turning. To validate if you are in a climb/decent, the pilot can look at the vertical speed indicator and altimeter. If they show you going up or down and the attitude indicator agrees, then you are in fact changing altitude. To validate if you are in a bank, the attitude indicator would be cross-referenced with the turn coordinator and the Directional Gyro. If they all three show you are turning, then you are turning. In level flight all of the instruments stay neutral.

The systems are designed to be redundant from how they are powered as well in order to minimize the chance that if one fails you will have others to reference. The turn coordinator, for example, is generally electric whereas the attitude indicator and the Directional Gyro's are generally vacuum. This is where it gets interesting...

The primary source for bank is the Directional Gyro and it is also powered by the same system that powers the attitude indicator. If a vacuum system fails, then chances are the pilot may not realize it until it is too late. It can be quite confusing when two of the key instruments fail. They also don't simply stop. They gradually slow down and become unreliable. The DG and AI will fail at the same rate and pilots often attempt to follow them. This causes what is known as a graveyard spiral where they fight to keep they gyros neutral when they are actually banking and climbing/diving at an increasing rate. There are many cases where a pilot does this until they enter a spin and drive out of control into the ground.

Scary as this is, this type of accident is more common than most pilots would like to acknowledge. And most pilots do not practice partial panel often enough to detect this type of failure. In fact, the ones that do train for it are expecting the failure. VAC failures can happen at any time without warning, which is what makes this particular type of failure deadly. If an electrical, engine or any other failure happens it is generally instantly obvious to a pilot since many other critical components are impacted as well.

Knowing that VAC failures are one of the top reasons why qualified, high-time pilots crash in IMC got me thinking about VAC systems. As I started researching why VAC pumps were so unreliable, I discovered that back in the early days this was not the case. In fact, from about 1937 when engine driven VAC pumps were invented to 1970 when dry pumps were introduced, VAC failures were nearly never a cause of an accident. When I dug a bit deeper I found some interesting information...

Changing Paradigm
Back in the 1970's a marketing campaign came out pioneered by Airborne that convinced pilots and new aircraft manufacturers that there was a newer and better VAC pump available. The new design was what they called a "dry pump. " A dry pump is virtually identical to a wet pump except they use a fine carbon fiber spinning ring that touches the outside metal housing of the pump to generate a vacuum. Since the carbon touches the outer ring it is able to seal without the need for engine oil.

The Airborne FUD (Fear Uncertainty and Doubt) campaign was based on the fact that the old wet pumps were in use since the late 30's therefor they were obsolete. Their spiel was that the dry pumps were cleaner, lasted longer and cheaper without providing any supporting information to substantiate this claim. Turns out that,
  • the cheaper part was true if you didn't count having to buy 4+ dry pumps over the same lifespan as one wet pump.
  • they are somewhat cleaner from an oil mist perspective but they throw out carbon dust that will destroy vacuum gyros.
  • the longer lasting claim quickly proved not to be the case, but this was after most wet pump companies went out of business.
Where are we today?
A typical dry VAC pump will last 250-500 hours, which on the surface isn't too bad if you include the VAC pump into a preventative maintenance program. However, the core problem is that when they fail they fail instantly. There is generally no warning of a system on the verge of failure until it fails. From a preventative maintenance perspective, some VAC pump manufacturers have an inspection hole that can be used to check the quality of the carbon wheel. This is a plus at annual time, but it isn't practical to open the cowl and check the pump during preflight. Accordingly, turning the prop backwards with a dry pump installed can destroy the fine carbon veins. This is a common occurrence since line staff and others that have access to the airplane. The prop is often moved without the pilots knowledge. For example, simply hooking up a tow bar often requires the prop to be moved.

The only effective maintenance program on a dry pump is to regularly change the pump. Replacing the pump is easy enough, but when? Every 200 hours, every 500 hours, after the system fails, etc? Waiting until a failure isn't wise since they throw out carbon powder that will likely destroy the gyros in VAC instruments. The cycle that many owners complain about is that the pump fails and is replaced. Then the VAC instruments fail sometime later for no apparent reason and are replaced. Then the VAC pump, then the instrument, etc. When a VAC pump goes, the entire system should be disassembled and cleaned, but no one ever does this because of the cost and time involved. The end result is paying thousands of dollars in order to save a few bucks while simultaneously taking a chance with the safety of the soles on board and countless people on the ground.

Reintroduction of Wet Pumps
For the past 25 years or so, wet pumps have not been manufactured by anyone. They have been available, but only as a rebuild. Many of the dry VAC pump companies have also gone by the wayside primarily due to litigation. Even Airborne that started all of this doesn't make dry pumps anymore.

Parker, a well known Dry VAC pump manufacturer has even authored an FAA Aviation Safety Publication titled "The Silent Emergency, Pneumatic System Malfunction." The publication downplays the severity of the issue by saying only two planes a year crash out of control due to VAC failures; however, they clearly recommend not flying in IFR conditions unless you have a backup VAC system on-board. The first section of the publication tells the tell, "All too often a pnumatic system failure leads to a situation where a pilot is forced into "partial panel" instrument flying that he or she may not be fully prepared to handle. A number of these accidents occurred to instrument rated pilots who recognized the pneumatic system failure, flew on partial panel in Instrument Meteorological Conditions (IMC) for some period of time, and then lost control during high task-loads, such as during an instrument approach."

When manufacturers put out flight safety publications warning against the use of their products, the NTSB data clearly shows that many IMC accidents are caused by VAC failures, it is common amongst pilots and mechanics that VAC systems are unreliable, etc., I think the jury is in -- Dry pumps have more than proven themselves not to be something you want to bet your life on, yet they are being installed by unknowing pilot/owners every day! Fortunately, now their is an option...

AirWolf Corporation has been in business since 1963. They have seen the wet pumps come and go and the dry pumps fail time after time. They decided a few years ago to reintroduce the wet pump to the marketplace. They did so by acquiring Pesco, which was a former wet pump company and then retooling the factory with more modern equipment. The wet pumps of today are computer machined to 0.0002 tolerance out of a solid billet of 6061 aluminum. This manufacturing process is well beyond what was possible in the 40's. This tight tolerance coupled with a pump made out of a solid aluminum billet makes the new AirWolf Wet Pump virtually bullet proof. In fact, AirWolf is so confident in the design they offer a 10-year 2000 hour warranty!

I love the quote form the President of AirWolf that was printed in Aviation Consumer. "Our specialty is not inventing something from scratch, "Kochy told us, "but making something better."

Aviation Consumer Says it Best
Quote from Aviation Consumer Magazine, August 2007
"We have no reservation in saying that a wet vacuum pump is right for every airplane—it’s just better technology than the dry pump. But it’s 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 it’s 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. "

Wet Versus Dry
I agree with Aviation Consumer and I agree with the proven Wet pump in the field. There is no question in my mind that a wet pump is the best bang for the buck. Sure it costs more, but can you really put a price tag on the safety of you, your passengers and the countless people on the ground? Why take the chance? When your VAC pump fails, and it will, replace it with a wet pump. PERIOD!

Clearly the decision to move to a Wet pump is a "no brainier." I've got to change the oil and do some preventative maintenance anyway. Next posting I'll discuss the ins and outs of installing an AirWolf Wet pump in 36G...


Anonymous said...


please tell me something: how can the bad vacuum pump destroy the vac instruments when typically the pump is a SUCTION pump and? I mean the pump is sucking at the attitude indicator's gyro, and also through a filter!
I am asking because my Warrior had a vac pump failure the other day, and now that the pump is replaced the AI is tumbling like crazy for minutes after engine start ...

thank you!
alexis von croy
munich, gemany

Jim Kerr said...

Alexis, when a dry vac pump fails the carbon disks tend to throw out carbon dust. This dust makes its way into your instruments. Vac pups should be replaced before they fail or the entire system needs to be taken apart and cleaned prior to use.