Peter MacPherson:
At what rpm do people see their vacuum pressure drop below an acceptable
level(3.2?). I also have a standby vac with a low vac pressure red warning light. When I reduce power to 1200-1300 rpm the vac pressure drops off and the red light comes on. I thought in the past the pressure would hold until around 1000 rpm. Does this seem to be working properly?
Editor:
Depends on the age of the pump. When they get closer to 1,000 hours, they begin losing low-RPM output. It is common for them to go under 3.27″ even at an RPM level of 1,500, if they are becoming high-time pumps. Contrary to popular belief, 3.27″ will run a pair of most gyros. My personal rule of thumb is a minimum of 4″ of suction, at the minimum power settings used during an instrument approach. In my Sierra those numbers are 2,450 RPM and 18″ of manifold pressure (while gear up). They will still last for quite a while when the vacuum begins to decline. I believe them to have the slowest rate of wear in the last couple of dozen hours before a vane finally breaks.
Our first pump (an Airborne 211CC, on the plane when we bought it) failed on the ramp in Vero Beach, during taxi. A log search showed that it had run for 1,400 hours. I changed the second one at 1,000 hours, prior to failure. It was still producing 4″ at 1,500 RPM when I changed it. Since you have the standby vacuum system, you should have less risk related to an in-flight failure (if you understand how to use it and have practiced it). I would not run one past 1,000 hours, though.
Did you notice that the new dual-chamber pump was recently pulled from the market? They actually had an incidence of simultaneous failure, something they thought would never happen. They pulled all the product and closed their doors. Didn’t matter to me; I didn’t think the extra length would fit on my Sierra, without making oil filter changes much more difficult.
My personal opinion is that your pump is OK; those numbers pretty much match my own, and mine probably now has 900 hours and is ten years old. I’ll be replacing it next year. There seems to be a fairly rapid initial decline in low RPM vacuum, then the rate of decline tapers off. If there are no adverse operating conditions (excessive heat, or getting oil or solvent in the pump or drive), and if the installations are properly done (meaning good air lines, previous replacement prior to failure, or thorough cleaning of lines following a failure), I am confident in the Airborne pumps for 1,000 hours.
Having said this, I do believe in changing them before they fail, for a number of reasons. A key reason is that this is the surest way to prevent premature failure of the next pump (no debris in the lines). Other advantages are personal convenience, higher likelihood of a more reasonable cost, and safety.
Pump life can be fickle, and there are no guarantees. I have no way of knowing the history of your particular installation. I do believe that the people who seem to have premature failures are the victims of bad installations and previous in-service failures. One way you can decide on replacement time is checking the logs to see if the previous pump failed in service; and if so, what the log says about what actions were taken to clear the lines before the current pump was installed. You can also confirm that there is good cooling airflow in the vicinity of the pump. A key criteria is whether you fly hard IFR, and whether you have standby vacuum (which you evidently do). A final comment on this subject; I have no significant experience with other than Airborne pumps. More curren research has convinced me that my next vacuum pump will be the AEON piston pump..
Regarding how you use the SVS-Precise Flight Standby Vacuum System. Your POH should say to NOT use it during takeoffs. You could potentially use it during low-power approaches, but if you suddenly had to do a full-power go-around, you’d be in the same scenario as a takeoff. You’d also play heck trying to retain vacuum during power changes, at a busy time. These constraints are due to how the SVS system works. It is intended only to help you recover from a cruise-flight or approach failure, while in IMC, and help you get safely on the ground.
The SVS system is at its best in planes with a constant speed prop. The way you get vacuum from it is to immediately go to max RPM, then check the manifold pressure. Let’s say it is 24″. You then reduce the throttle to 20″ of MP. The 4″ difference will give you a vacuum reading of 4″ on the vacuum gauge. If you have a fixed pitch prop, you have to reduce RPM (throttle) until the vacuum gauge reads 4″. Both of these actions will reduce flight power. In my case my Sierra can still get 90 knots IAS at ten thousand feet with the SVS system on. It is the loss of power that results from the use of the SVS that makes it inappropriate for routine use as opposed to backup use. If you use it at a time when you are making power changes, it can also get busy trying to ensure at least 4″ of vacuum, unless the change is a continual throttle reduction. The final factor is that with the engine pump operating and producing vacuum, you cannot really tell whether it is the pump or the SVS that is “doing the most work”. When the SVS is installed, the pump hose is temporarily disconnected and the plane is taken up for a calibration flight. This flight is to generate the operator guide that goes in the POH, giving instructions for throttle and prop position during SVS emergency use, at altitudes up to 10,000 feet. Those numbers are invalid if the pump is connected and working. You can’t know where the automatic check valve is positioned; so how would you know where to set the controls?
The best thing to do is to understand how the system works, and to be prepared to use it immediately if needed. You should have a red light that comes on if the pump should fail. You should see this light come on with the Master, prior to engine start. If that light should come on during flight, and you are in VMC, there is no emergency. You can take your time. If you are in IMC, just remember that you have to immediately reduce throttle to get 4″ of MP. That will ensure that your gyros don’t spool down. Then you’ll be able to safely maneuver if needed, while choosing your next course of action for your conditions. Your #1 priority should be to (preferably) get to the nearest VMC conditions while at a safe cruise altitude. Only if you are desperate should you fly an approach while on the SVS. Especially a precision approach, due to the possible sudden need for a missed approach, with resulting low altitude maneuvering toward the MAP. This isn’t because it can’t be done; it should be readily possible. It’s because of the variability of operation and the relative lack of understanding of the system by most operators. It is always the safest action if you can get to VMC before maneuvering near the ground, when your equipment is not 100%.