I have had two vacuum pump failures, in flight, in twenty years of flying. The first was in VMC conditions and was a non-event. The second one occurred on my very first IFR training flight in actual IMC conditions.
We had just leveled off from Santa Rosa Airport (STS) when my CFI prompted me that something didn’t appear right. I scanned the gauges and noticed the suction gauge was reading zero. At the same time, my attitude indicator was doing a lazy roll to the left, while my other gauges and whiskey compass gave no indication of a turn. Cardinal 34777 had a vacuum system failure!
At the time my first thought was, “How cool is this? I’m flying partial panel in IMC conditions on my very first IFR training flight!”
My flight instructor was very calm. He said, “We’ve got a number of options, let’s just think them through and we’ll decide what to do.” We discussed the options and then decided to fly the ILS 32 approach back to STS.
“This will be good practice for you,” says my instructor.
My recollection of flying the approach was that it didn’t seem difficult. I was probably coached and prompted more than I realized.
In my case the identification of the vacuum system failure was fairly quick and obvious. However this is not always the case.
In hard IFR, with a heavy work load and a slowly failing vacuum system, the problem can be far more insidious. If the attitude indicator begins to tumble slowly the pilot may inadvertently begin to follow the erroneous indication.
As he chases the slowly turning and descending horizon line he is actually placing the plane in a descending spiral. When he cross checks the situation with the other instruments, nothing looks “right” anymore. A further delay as a result of this confusion may result in an unrecoverable aircraft. It is every IFR pilot’s worst nightmare.
After my brush with an IFR vacuum failure I decided to delve into the details of partial panel flying. The following is a summary of my research.
“Partial panel” can refer to any loss of one or more of the six basic instruments in a steam gauge panel.
A blockage in the pitot/static system can cause erroneous readings on the airspeed indicator, the VSI or the altimeter.
An instrument failure or electrical problem can cause a loss of the turn coordinator.
A vacuum system leak or vacuum pump failure can cause misleading or erroneous indications by the attitude or heading indicator.
The most critical of the above noted failures is the vacuum system failure. This is because the attitude indicator and the heading indicator give direct and immediate indication of aircraft control input and response. Climbs, descents and turns are immediately indicated on these instruments. They are often referred to as the “control instruments” for this reason.
The other four instruments in the six-pack are the “performance instruments”. They give an indication of how the aircraft is performing after a control input has been made. This is a critical distinction. Due to the nature and response times of the pitot/static instruments, there is a significant lag between the time a change has been made to the aircraft attitude or airspeed and the time the effect is registered by these instruments.
When flying in VMC this lag time seems fairly short and inconsequential. However, when flying in IMC after a vacuum system failure, the lag in response time of the pitot/static instruments must be clearly understood in order to maintain aircraft control.
Vacuum Pump Failure
The Bullet Points of Partial Panel
- Frequently check the suction gauge during flight
Suction from the vacuum pump drives the attitude indicator and the DG or heading indicator. A failed vacuum pump means loss of pitch and bank control as well as heading indication due to:
- Loss of attitude indicator
- Loss of directional gyro
You must make control inputs based on performance instruments. The performance instruments include:
- Airspeed indicator
- Altimeter
- Vertical speed indicator
- Turn coordinator
The pitot / static instruments - airspeed indicator, altimeter, vertical speed indicator all lag behind attitude changes. After you make a control change there is a lag before the change will register on these instruments:
- The turn coordinator varies with airspeed and gives indirect indication of bank angle.
- Each movement on partial panel must be smooth and gradual.
- A standard rate turn is 3 degrees per second: 30 degrees in 10 seconds.
- On partial panel use 1/2 standard rate turns on approach only.
As soon as you make a control adjustment - back it up with a secondary instrument.
- After initiating a turn with the TC, verify it is a level turn with the altimeter
- During roll out the TC will indicate a turn in the opposite direction
- When you stop a turn and level the wings back it up with the compass, altimeter and VSI
- Use the GPS to help you back up the turn heading
On initial failure, first do nothing. Take a deep breath. Asses the situation.
Next reduce power, slow the plane down and ensure the wings are level.
Keep the plane properly trimmed.
Making Turns on Partial Panel (Vacuum Failure)
- The magnetic compass numbers are backward compared to the DG and the map. Look at the VOR or GPS to decide which direction to turn
- To determine the number of degrees to turn, count the 10 degree increments using the DG face, if it is not immediately obvious.
- To determine how long to make the turn; divide the number of degrees by 3. This will give the number of seconds to remain in the turn. I.E. 33 degrees / 3 deg's. per sec. = 11 second turn.
- As you roll out from a turn be aware that the TC will indicate a turn in the direction of roll
- For turns of 10 seconds or less just count the time in your head
- Use the VOR to track your course. Make small corrections and see how the CDI moves
Request a No-Gyro Approach
- If in IMC during a vacuum system failure and cannot fly to VMC, then requesting a no-gyro approach from ATC may be the safest way to get on the ground
- ATC will instruct you to turn left or right, using the turn coordinator, and then tell you when to stop the turn.
- ATC will expect you to make standard rate turns and to start and stop turns as soon as you are instructed.
- ATC will notify you when you are on final approach and you will be expected to make 1/2 standard rate turns.
Altimeter Failure
- Practice climbs and descents at a specific vertical speed, say 500 FPM.
- In the event of altimeter failure you can time the descent in FPM. For example:
- If the altimeter fails at 5000 feet and you need to descend to 1000 feet, then a 500 FPM descent will require 8 minutes.
- Do not chase the VSI. It lags behind control inputs, so make a.small change and wait for the indication to stabilize
Unusual Attitudes
Generally speaking there are two types of unusual attitudes:
- nose low attitude
- nose high attitude
Nose Low Recovery
In a spiral dive:
- First reduce power
- Then level the wings
- Then raise the nose
- Raising the nose first in a spiral will tighten the spiral and increase the rate of descent
Nose High Recovery
In a climbing turn approaching stall:
- First increase power
- Then lower the nose
- Then level the wings
Summary
Partial panel flying is manageable and not difficult. Frequent practice and a close monitoring of the suction pressure gauge can make this potential problem a non-event. Of course, the best solution is to have a back-up vacuum system or instruments that can provide direct indication of pitch and bank.
