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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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Top ten reasons why you should never learn how to fly

Flying? Boring! Why would I ever want to do it? Aside from being expensive and time consuming,  it’s also pointless. Why would I want to shoot around the sky in a metal tube?  It just seems like a poor use of time. Seriously, only those with poor judgement would consider getting a pilots’ license.  There are just so many reasons why you should never do it, but here are the top ten that I could think of.

1.  The view sucks.  Why would I want to see the world from 3000 AGL?  I have such a hard time picking out my house from the plane, it’s so much easier if I’m driving around in my car.  The world just looks so big from the air and it’s really not that interesting.  I’m just not interested in seeing the bigger picture.

These guys are always bossing us pilots around. You'd think they own the airspace or something.
These guys are always bossing us pilots around. You’d think they own the airspace or something.

2.  Airports are boring.  Nothing interesting happens at airports. Seriously, flight schools talk about teaching you soft field landings and precautionary/forced landings, but never actually let you do them, always forcing you to return to the airport. How about some danger? I’ve seen Top Gun – I’m ready!

3.  Air Traffic Controllers are bossy.  They constantly tell you what to do and they talk so fast that you can barely understand them most of the time.  And they always tell you to switch frequencies and get upset if you don’t call them.

4.  Too many calculations. You really have to learn how to flight plan to learn how tedious and pointless it is.  Just point the airplane in the direction you want to go – how much harder does it have to be, people? Fuel, schmuel. I’ll just keep an eye on the fuel gauge like I do when I drive. What could possibly go wrong?

5.  No in-flight entertainment.  Unless you count your instructor, there is no real source of entertainment when you’re flying.  No movies, TV, music or anything. No hot coffee. The service stinks.  Why would I want to sit in an old uncomfortable 30 year old Cessna when I can comfortably stretch my legs out on the couch like a human being.  When I stretch my legs out in the Cessna I just end up hitting the rudder pedals, which causes yaw and I have to do more work to correct it.  Can’t relax in the thing for a second.

6.  Flight instructors are annoying.  They are always telling you what to do and bossing you around. It’s worse than ATC because you can’t really get away from them.  Once you leave the control zone you are free of the claws of terminal control but you can’t get rid of your instructor once you let them in the plane with you.  They really don’t know that much … How much can there possibly be to know??

7. Too much safety emphasis.   Do you know how long it takes to prepare to actually go on a flight that lasts less than an hour? About two hours. Checklists, meetings, briefings, log books, journey log books, sheesh. So much paperwork and so many safety checks. I mean, did you know when you’re at the hold short line that you have to check your engine is operating? It started, so why do you need to check it again? Obviously it’s working and the plane is ready to go.  If it wasn’t it wouldn’t start. Obviously.

8.  Trainer planes are old.   Trainer planes are so old, I think they must have been built when dinosaurs roamed the earth.  They are loud and uncomfortable.

9.  It’s too hard.  The instrument panel in your car has only a few sources of information: speed, fuel quantity, engine temperatures and some have a tachometer.  The most basic airplane panel has six highly confusing instruments which are really hard to understand, ever mind all the other engine instruments, radios, direction finding equipment, navigation tools, approach systems, and the little floaty thing on the dash.  There are so many maneuvers, attitudes and movements too learn, it’s just seems like way to much work.

10.  It’s scary.  The whole concept of flying just seems like a bad idea.  There are just too many planes of movement.  It’s not that rewarding, and not really fun to be in control of the thing.   Constant briefings, meetings, exams, preparations, paperwork and safety checks.  Soaring through the air?  Make a career out of it? Seriously why bother. I’d rather just sit on the couch eating chips.

We hope you had a good laugh reading this.

Sarcasm aside, ever try to talk yourself out of getting a license?  So many reasons. The fact is that flying is hard, committing wonderful and very rewarding.  Like many things in life, if it wasn’t hard, it wouldn’t be worth doing.   Challenge yourself and don’t give up.  Search your soul and if you discover flying is right for you, you will have the time of your life and you will not regret it!



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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?