From a pilots earliest days of instruction, the need to have an out gets drilled over and over. Typically, pilots are taught to have a strategy to get out of trouble if the course upon which you are proceeding becomes untenable. But the best-laid plans wont help you if you lose the tools to execute them.
Thats one reason why instrument training leans heavily on partial-panel skills. The accident record shows there are relatively few casualties to vacuum system failures during IMC, but pilots who survive them tell harrowing tales of near disaster.
Most light aircraft have vacuum-driven attitude indicators and directional gyros with only one source of vacuum – a pump mechanically driven by a gear in the engine-mounted accessory gearbox. While virtually all aircraft flown under IFR have an electrically driven turn and slip indicator or turn coordinator, pilots tend to run into trouble when the vacuum pump quits.
There are a lot of reasons why this happens, ranging from lack of partial panel proficiency to failure to recognize the problem when the vacuum-driven instruments start to wander. In both cases, the pilot is unable to sort out the conflicting data presented.
Getting on the soapbox for a moment, I think a good session of partial-panel flying, including a nonprecision approach to minimums, is essential every six months for all light plane instrument pilots who dont have the luxury of flying airplanes with substantial backups. However, the fact is that reasonably priced technology exists that would prevent most partial-panel operations caused by failure of the vacuum system.
Easily the most important item is gyro-stabilized attitude information. The wet compass is a poor substitute for a DG when it comes to precise flying, but itll get you there if you dont care about style points.
Attitude indicators themselves generally dont fail without substantial warning, particularly vacuum-powered AIs. Therefore, the obvious solutions for avoiding loss of an attitude indicator is to have a single AI with redundant power sources or redundant AIs, each powered by a different source. Because vacuum and electric are about all there are, it would seem that the major options are a single vacuum-driven AI with a backup source of vacuum, a single electric AI with redundant electrical power or two AIs – one primary and another secondary, powered by different power sources.
One Vacuum AI, Two Vacuum Sources
There are three feasible ways to provide a second source of vacuum. You can install a second vacuum pump, tap vacuum off the engine intake manifold or resort to an old-fashioned venturi. In any case, youll need some sort of sensing system that identifies loss of vacuum pressure, preferably with an automatic switch such as a shuttle valve that flips over to the backup source if the primary source fails.
There are several ways to provide a second vacuum pump. One is to add another mechanically driven pump driven off the alternator belt. This alternative will require some creative engineering and a field approval, as I know of no STCd systems of this type. The weight penalty is minimal and the system has the advantage of using a different means of driving the backup pump (belt off the crankshaft vs. the accessory gearbox).
Another is to use one of the commercially available electric-powered backup vacuum pumps. These weigh about 10 pounds and cost around $2,000 plus installation. Their biggest advantage over the first alternative is the off-the-shelf ease of installation.
Finally, you can use a standard vacuum pump driven off any available empty drive pad on the accessory gearbox. This system is probably the simplest to install, but is limited to the extent that not many light singles have empty drive pads waiting to be filled.
The manifold tap systems like the Precision SVS are very reasonably priced, STCd, and pretty easy to install. However, they have a couple of limitations that make them questionable choices for light singles, especially those with fixed pitch propellers.
These systems work by harnessing the differential between ambient air pressure and the reduced pressure in the intake manifold, and require about 3-5 inches of pressure differential. The problem comes if you are in an aircraft with a fixed pitch prop above 6000 feet or so.
As ambient air pressure drops, manifold pressure must also be decreased to keep the gyros spinning. Above 10,000 feet, you may be talking as little as 15 inches. With a fixed-pitch prop, youll already be working with fairly low manifold pressures at cruising altitudes.
For example, a 160 hp Lycoming O-360-D2J (as installed in a Cessna 172) can pull about 75 percent power at full throttle at 7,000 feet. However, if you need to reduce the throttle enough to produce a standby vacuum source, power output drops to about 50 percent. While this will keep you flying, it will not let you climb – and things get worse as you cruise at higher altitudes.
You should expect to be unable to climb at lower altitudes and be unable to maintain altitude much above about 7000 feet. This means trouble if youre over high terrain or have some other reason you need to climb or stay high.
Aircraft with constant speed props have it easier, as the prop governor allows you to pull the same power at reduced manifold pressure by increasing prop RPM. Thus, if you are cruising at 7,000 feet at full throttle (about 23 inches) and 2300 RPM, you can get the same power at 19 inches by pushing the prop up to 2700, giving yourself adequate differential to run the gyros without sacrificing performance.
However, if you do make an approach and miss, applying full power to go around will zero your pressure and run down your gyros just when you need them most – low, slow, and in the weather.
Another drawback to manifold tap systems is that most draw off the intake for only one cylinder. That results in a gross deviation in fuel/air mixture and flow to that one cylinder, which can cause very high combustion temperatures in that cylinder. In extreme cases, it can lead to damaged valves, valve guides, rings, or pistons if the operation is extended or conducted under severe conditions. While any pilot would rather have to replace a valve guide than lose control in IMC, the fact is that there are other options which do not require such a choice.
The venturi system used by many classic aircraft is another option with certain advantages. It has no moving parts, doesnt care what your manifold pressure is, and doesnt even care if the engine fails. It also has a number of disadvantages, such as drag and a propensity to be a real ice-catcher if you find yourself in such conditions. Electrically heating a venturi takes about two to three times what a pitot heater does. Finally, its ugly.
Electric AI with Redundant Power
The next choice is to replace your existing vacuum AI with an electric instrument. Of course, if your aircraft gets its electrical power from an alternator driven by a belt off the crankshaft backed up by a battery, this puts you at the mercy of the electrical system, but there are some mitigating factors here.
If the electrical problem is an alternator failure, youll still have electrical power for a while – until the battery dies. However, a recent study of 139 instrument-rated private pilots by Paul Craig at Middle Tennessee State University using a Frasca 141 simulating an alternator failure in low IMC showed that most of the pilots were unable to end the flight safely before the battery ran down. With an electric gyro, the time available before the lights go out would be even further reduced unless you also make provisions for a battery with extra capacity.
In addition, electric AIs cost two or three times as much as vacuum AIs, even before you pay for the installation and FAA certification process, as field approval will be required unless your airplane has an electric AI as an original option or theres an STC available for sale. You can also cut your losses by leaving the vacuum DG, so your two primary gyro instruments are powered by separate systems, though the value of a working DG when the rest of the panel is dark may be of little consequence.
The way around this is to develop a truly redundant electrical system like those installed in transport category aircraft. Aeroelectrics Bob Nuckolls has a number of ideas on how to accomplish this. (See www.aeroelectric.com). He starts by going to an all-electric panel, completely eliminating the vacuum system. This opens up the vacuum pump drive pad, allowing installation of a pad-mounted alternator.
B&C makes such units in 8 and 20 amp sizes, Nuckolls says. Paired with a small battery, 6 amp-hours or so, for stabilization, the second alternator will certainly run enough goodies to get you home comfortably.
He suggests installing a system in which minimal panel lighting, the turn coordinator, the primary nav/comm and the electric AI are on an essential bus driven by a diode feed from the main bus. In event of main power failure, the essential bus is switched from the main bus to the backup alternator.
Nuckolls goes on to suggest that if the second alternator is of the larger, 20-amp size, he would completely redesign the electrical system into two major bus structures, each supporting its own 12-amp-hour battery. A cross-tie connector allows one alternator to feed both busses if the other one quits, as well as allowing both batteries to be used for engine start. At all other times, the busses remain completely independent.
While it might seem that Nuckolls idea would carry weight penalties, thats not necessarily true. Since the airplane now has two batteries that work together for cranking the engine, neither needs to be as large as the original single battery. Also, since there is a backup alternator, you dont have to worry about the battery having to carry a major electrical load for a long time if one alternator fails. That further reduces battery requirements.
Since all the batteries are really doing is providing stabilization for the alternators and enough power together to crank the engine for start, the combined capacity of both batteries can be reduced to 10 amp-hours or less, possibly as little as 6 amp-hours. With the weight of the second alternator offset by the removal of the vacuum pump – the 8-10 amp alternator will actually be a bit lighter – this would result in a substantial weight savings over the traditional systems.
While this may sound like a pie-in-the-sky setup, its not. This is pretty much the setup Cirrus Design opted for in the SR22. It has two alternators (60 and 20 amps), and two batteries (one 10 amp-hour and the other a dual-unit 7-amp-hour system). The big alternator feeds the main bus, the smaller unit feeds the essential bus, and the two busses are interconnected by a pair of 50-amp fuses and a diode. The AI is electric and the DG is replaced by a Sandel EHSI. You can read all the details on Cirrus web site (www.cirrusdesign.com) by clicking on AIRCRAFT and the SR22 POH.
Nor would such a system necessarily be cost-prohibitive for an owner who was already planning major airplane rehab. For most of the components of the system, the cost of the replacement items would be fairly competitive with the current electric-plus-vacuum system. The major hurdle for an owner upgrading his aircraft to this configuration would be the certification paperwork. Since no one has done any STCs like this for older aircraft, you would have to go the field approval route with the local FSDO, and thats a bit of a crap shoot. Something that one FSDO would happily approve might get a big thumbs down in the next district. A bit of FSDO-shopping can help this, but the FAAs lack of standardization in the Flight Standards division is a barrier to making this work.
Backup Electric AI
The other way to make this work is an exercise in simplicity. Install a backup electric AI. Most panels have the room to squeeze in a 2-inch electric AI, preferably where its easy to scan in case you really need it. Its still a good idea to either install a vacuum-failure warning light or replace your AI with one incorporating a vacuum failure warning light so you know when to stop looking at the big vacuum AI and start looking at the little electric AI.
The FAA has issued guidelines for what constitutes an acceptable backup attitude indicator, which can be found in Special Airworthiness Information Bulletin CD-01-29.
The FAA recommends that the backup indicator be powered from a source independent of the pneumatic generating system, which is certain enough with an electric gyro. It suggests the system should provide reliable operation for at least 30 minutes after total failure of the electrical generating system. It should operate independently of any other attitude indicating system, it should operate without pilot intervention even if the electrical generating system fails.
Personally, Im not sure the independent 30-minute power source is necessary, since youll still have your primary vacuum AI if the electrical system fails, but there are units out there that incorporate such capability – at a stiffer price.
For many years, instructors and safety gurus have said that the combined fragility and criticality of the standard dry vacuum pump is a danger to be reckoned with for all pilots who fly IMC in single-engine aircraft. The accident record is rather spotty in this regard, yet the technology is clearly available to do better.
Newly designed production planes hitting the market are applying these ideas to provide redundancy in the critical area of flight attitude information instruments. Even something as simple as installing a vacuum-failure warning light may be enough to help you tell when its time to go to the old needle, ball and airspeed mode.
But any way you look at it, there is no longer any good excuse for finding yourself without knowing which way is up.
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-by Ron Levy
Ron Levy is director of the Aviation Sciences Program at the University of Maryland Eastern Shore.
