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Instrument flying: flying with a partial panel

cockpit 1957 beechcraft airplane

The complexity of  flying on instruments increases when we simulate a vacuum failure. We loose one especially critical instrument necessary to our flight attitude coordination. The Loss of this instrument in flight can certainly and very quickly and easily turn into a life and death situation.

The purpose of partial panel training

The goal of partial panel simulation is simple: what would happen if you were to have a vacuum failure in the most critical time, when you were in IMC or flying at night? The reason for learning essential basics of instrument flying, including emergencies such as partial panel, doesn’t have to be so complicated as to just needing it to earn your IFR rating for airline flying. 

How the vacuum system works

The vacuum system is operated by a venturi which is usually engine driven. The change from higher to lower pressure drives the gyros, so these require some time to spool up and are only accurate after takeoff.  

The heading indicator and attitude indicator are vacuum system powered gyros and the turn and bank indicator are electrically powered. So in the event of a vacuum failure, you’ll be able to use your turn and bank indicator to assess when you are wings level and coordinated.

How can we end up with a partial panel in real life flying?

Since in airline flying you’d never encounter this situation, you can experience a vacuum failure at the worst possible as a private or bush pilot, or a pilot for a smaller operation that does bush flying in remote areas. There can be pressure to complete a job, pick up passengers, or get people to a certain destination. You know the weather is going to deteriorate but you decide to go anyway. You fly into the front which has come earlier than forecast and end up in a situation where you are pushing the weather.

Deteriorating weather

Picture you’re on a night cross country flight with little to no great reference to the ground. You’re essentially flying on instruments. Or, you’ve departed during day VFR with a sketchy forecast, and you’ve inevitably flown into an area with low ceilings and decreasing visibility. It starts slowly at first, and before you know it, you can’t see the ground, and you don’t know which way is up.  If the worst was to happen and your vacuum system loses suction at this time. You’ll be in deep, trying to keep the airplane under control while trying to figure out what the heck you need to do to get yourself out of this situation.

The first thing you do, of course, is be prepared for this type of worst case scenario by practicing these difficult situations under the hood or better yet, in the simulator. You can even practice at home. Have your instructor create a scenario for you where you are flying to an area with a less-than-ideal forecast tracking different VOR radials and NDBs, and along the way simulate slowly diminishing visibility until you are forced to divert. Enroute to your diversion aerodrome you loose your vacuum system, and are forced to fly without your AI and HI. You need to get to your airport and out of this mess. 

Cessna 182 in northern alberta
Cessna 182 stuck at a snowy airfield in Northern Alberta.

1. Don’t panic – fly the airplane

The first thing you do if this happens to you is to remain calm, and fly the airplane. Remember to always aviate, navigate and then communicate, in that order. Always focus on flying the airplane before you do anything else. This is especially true when you’ve found yourself in a low visibility situation with limited instruments. 

Focus on the instruments that give you the information you need, and start your scan. In the case of full panel flying, this is a lot simpler because you have your attitude indicator at the center of your scan which gives you your most critical information: the position of your airplane against the horizon. Are you nose up or nose down, and are your wings level or are you in a turn?

Start your scan

When you lose your vacuum system, your gyros, the heading indicator and attitude indicator will be immediately unreliable. The major challenge with this is that these two instruments, particularly the attitude indicator, are at the center of our scan. So we have to quickly develop a new method. 

The main concept continues unchanged, you continue to control the aircraft with the formula attitude plus power equals performance.  The difference is now you have to look at other instruments to get this information. When flying without an attitude indicator, you must determine your pitch by primarily referencing your airspeed indicator, and verifying it with altitude and vertical speed indications. 

Control Instruments

Attitude: Airspeed Indicator

Referencing your airspeed indicator for pitch is challenging but doable and requires significant practice to master. My instructor set up a scenario in the sim where my vacuum system failed in cloud while on a low-level diversion to Red Deer. I flew this route a few times and found it took a few minutes to organize the scan before I got the aircraft into a reasonable state of control. The important thing is not to chase the instruments. I did this at first, and found my airspeed all over the place, and then my altitude started to fluctuate and I descended to only 500 AGL. 

This happened because I was not allowing the airspeed to stabilize. A certain attitude will give you a certain airspeed. Let it stabilize and reference your altitude and VSI to ensure you’re at a stable straight and level attitude. 

Turn information: Turn and Bank Coordinator

Use the turn and bank coordinator to verify that you are wings level. Use the magnetic compass to verify the heading has not changed. Do not fly heading via the magnetic compass, it’s too confusing. The compass works in the opposite direction to turn. So unlike a heading indicator, you turn away from the heading you want to go to, the opposite response that makes sense. The compass also has a significant amount of lag. It’s only reliable to verify that we are on the proper heading, but not looking to it as a control instrument. 

Your performance instruments

The performance instruments help you verify the impact of your control inputs are or aren’t what you want them to be. In partial panel flying, they are always attitude plus power equals performance:

Attitude + Power = Performance

Control Instruments + Power = Performance

Airspeed Indicator + RPM = Outcomes shown on the VSI, Altimeter and Magnetic Compass

Turn and Bank Coordinator + RPM = Outcomes shown on the VSI, Altimeter and Magnetic Compass

2. Navigate 

Find out where you are by using VORs, NDBs, GPS or ideally combination of those. You can also ask for vectors. This of course bring us to:

3. Communicate

Let air traffic control know you’re in an emergency and ask for help.

Next find out how to use rated turns to get yourself out of cloud and into an airport. Executing a timed turn is a critical skill and becomes very important during partial panel flying.


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The ‘six-pack’ flight instruments: gyroscopes

Continuing on our review of the ‘six pack’ of flight instruments from the instruments that are powered by the pitot-static system, below we review those that are gyroscopes.

A gyroscope is a rotor or spinning wheel rotating and high speed,  and exhibits two fundamental characteristics upon which all practical applications are based.  These are:

  1. Gyroscopic intertia –  or rigidity in space. This is the tendency of the rotating body to maintain it’s plane of rotation if undisturbed.
  2. Precession: This is the tendency of the rotating body, when a force is applied to it at a point perpendicular to the plane of rotation to react as if the force had been applied 90 degrees in the direction of rotation

The three gryroscopic instruments are:

  1. The heading indicator. The main instrument we use to detect heading of the aircraft.  Only operates when the engine is running.  It runs off a vacuum system so we have to adjust it to the magnetic compass every time we fly. Frictional forces in the gyro bearings cause it to precess, resulting in a creep or drift in reading approximately 3 degrees every 15 minutes.
  2. Turn and bank coordinator, sometimes called the needle and ball.  The needle shows the direction and approximate rate of turn. The ball shows the amount of bank in the turn and whether there is any slipping or skidding. The ball is controlled by gravity and centrifugal force.  In a coordinated turn, the ball will be in the center as the centrifugal force offsets the pull of gravity. The instrument reacts to yaw but can be used for roll control since the aircraft yaws when banked.  It can show a rate one turn which gives us 3 degrees per second or a two-minute turn.
  3. The attitude indicator. Modern attitude indicators have virtually no limits of pitch and roll and will be accurate indicate pitch up to 85 degrees, and will not ‘tumble’ in 360 degree rolls.

The instruments are typically powered by the vacuum system and an electrical system for redundancy in case one of the power sources fails.  Often the heading indicator and attitude indicator operate on the vacuum system while the turn and bank coordinator is electrically operated.

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The “six pack” flight instruments: pitot-static

Flight instruments on a Cessna 172

Let’s do a review of the six main flight instruments: 

Detail is provided, of course, there is so much more we can add here!  The most important and basic flight instruments have remained the same for a long period of time, and are called the ‘six pack’.  Three of them are connected to the static port system which measures outside barometric pressure and the pitot tube which measures ram pressure.   The other three are gyroscopic.

The Pitot Tube on a Cessna 172
The Pitot Tube on a Cessna 172

The pitot tube, located on the leading edge of the wing, and the atmospheric pressure in the tube is increased by the dynamic pressure due to the forward motion of the aircraft while in flight.  The static pressure port is not affected by turbulence or ram air pressures.

The three instruments connected to the pitot-static system are:

(1) Airspeed Indicator (ASI) – pitot and static source; it measures the difference between the pressure in the pitot tube and the pressure in the static system. When the aircraft is on the ground the two pressures become equal, in motion the pressure difference causes the aneroid capsule inside the indicator to expand, moving the needle on the instrument.

The ASI shows indicated airspeed.  Indicated airspeed can be erroneous because of air density, which depends on pressure and temperature, and position error, which is caused by eddies that are formed when air passes over the wings and struts. This is the uncorrected reading from the dial and calibrated airspeed is the indicated airspeed corrected for position error (and installation error). Equivalent airspeed is the calibrated airspeed corrected for compressibility – this applies mainly to high speed airplanes.  Next we have true airspeed which is calibrated airspeed corrected for pressure and temperature. Roughly, to correct calibrated airspeed we add 2% to the indicated airspeed for every 1000 feet of pressure altitude.  We can gain more accurate readings using our flight computer – the E6B.

(2) Vertical Speed Indicator, static source. Operates on the principle that there is a change of barometric pressure with a change in altitude.  Atmospheric pressure is led into the capsule but slowed by a calibrated leak from entry into the case holding the capsule,  and this pressure differential causes the capsule to expand or compress.  There is a 6-9 second lag before it will indicate the correct rate of climb or descent.

(3) Altimeter, static source. Since pressure varies from place to place and the altimeter set to indicate height above sea level at the departure point may give a false reading after the aircraft has flown some distance.  To correct for this, the altimeter is equipped with a barometric scale (inches of mercury) which allows to set the current altimeter setting. We get this each time we depart our airport and can get it enroute.  If we fly to an airport that has a lower pressure than the one we departed from and we don’t change our altimeter setting, we will read higher than the actual height of the airplane. Temperature differences will also cause erroneous readings since the pressure altimeter is calibrated to indicate true altitude in standard atmospheric conditions.  When the temperature of the air beneath the airplane is colder than standard, the aircraft is lower than indicated, and vice versa for warmer than standard temperatures (higher than altimeter reading) .

Here are what we can expect from a compromised static-port system.

Instrument Pitot Tube Blocked Partially Blocked Static Port Fully Blocked Static Port
Altimeter Not connected Under-read in climb, over-read in descent Freezes
Vertical Speed Indicator Not connected Under-read in climb, less than true rate of descent Freezes at 0
Airspeed Indicator Acts like altimeter. Over-reads in climbs and under-reads in descents Under-read in climb, over-read in descent Under reads in climbs and over reads in descents.

Read about the other 3  instruments that are gyroscopes: the heading indicator, attitude indicator and turn and bank coordinator.

Do you have any other specialty instruments in your aircraft?