Trust the Instruments?

In an aircraft there are three different types of instruments in the cockpit: There are the primary flight instruments, navigational instruments, and gauges. There are six primary flight instruments and these are always situated directly in front of the pilot. These instruments give information regarding the orientation of the airplane - direction, altitude, speed, etc. The navigational instruments give information regarding the geographical position of the aircraft. Examples would be GPS, VOR, ADF, DME, and a host of others. The gauges give information regarding the status of the aircraft's systems and engine. Even the simplest of certified aircraft are equipped with a Tachometer, Oil Pressure, Oil Temp, Fuel Quantity, Ammeter, and Suction Gauge (shows the health of some of the primary flight instruments, more on that shortly). More complext aircraft in addition will always have Cylinder Head Temp, Manifold Pressure, and Fuel Flow gauges. There are a host of other gauges airplanes can be equipped with to give a better indication of engine and systems health/performance.

Below is a picture of the primary flight instruments. Clockwise from the top left is the Airspeed Indicator, Attitude Indicator, Altimeter, Vertical Speed Indicator, Heading Indicator, and Turn Coordinator.

When flying in IMC (Instrument Meterological Conditions, aka "The Soup"), its commonly said that you should "always trust your instruments". This is because its instinct to naturally trust what your body tells as far as orientation goes. When you have no visual cues, its very easy to become disoriented, because the tools your body uses to determine what is up and what is down, can be very easily confused, and what you feel may be up could quite possibly be completely wrong.

As far as the say goes to "always trust your instruments", this is not always very good advice either. While it may be true that the instruments are right 99% of the time, they are still a mechanical fallable device. I think the saying should read, "Never trust your body, and be sure to question your instruments.". The two centre instruments in the picture, the attitude indicator and the heading indicator, are the two instruments that most intuitively show orientation, are actually powered by the same system - the vacuum system. The vacuum system is powered by an engine driven vacuum pump, which keeps a gyroscope spinning. The gyroscope is the heart of these two instruments, and if it stops spinning, these two instruments stop working.

Conviently, the vacuum pump drive shaft is built in such a way that it will shear off if something in it goes wrong. This may seem like a lame idea, but the idea is to protect the engine. Vacuum pump failures can and do happen - I experienced one once flying my Cessna 150. I was flying VFR naturally, so in those situations its more of a pain in the butt and a, "Dang, I wonder how much this is going to cost" problems. A vacuum pump failure through IMC is a little more serious, because that would mean we've just lost two of our better ways of determining orientation and direction, but all is not lost. In fact we can have any of the six instruments fail and we'd still have a way out. Our way out is called an instrument cross check.

The beautiful thing about the six primary flight instruments, is that many of them really have the ability to show two different kinds of information. The second type of information they show may be a little less intuitive, but the information is there. The concern of course is ending up in a spiral towards the ground and not knowing it, or banked at an extreme angle to cause a stall, resulting in a spin. This is where the instrument cross-check comes in.

The airspeed indicator as well as showing airspeed can also show us vertical speed, so can the altimeter. If we were in a nose down dive, the airspeed would be rapidly increasing. If we were in a nose up attitude, the airspeed would be decreasing. Make sense? If the airspeed is holding steady, we know we're holding altitude. The turn coordinator is our backup for bank information. The little airplane on the instrument will roll back and forth to indicate the rate and direction of a turn. The ball in the little glass tube below it will show slip or skid indications. So if the little airplane is level, and the ball is in the centre, we know we're holding our heading. So the airspeed indicator combined with the turn coordinator gives all the information we need for orientation, and our magnetic compass, which I didn't really mention (but we still have) can show us direction (just not as instantaneously as the heading indicator).

Any time we're flying by instruments, we are continuously doing an instrument cross check to verfiy the accuracy of the instruments. "Always question your instruments." If the attitude indicator is showing a bank to the left, but the turn coordinator is showing a turn to the right, or straight flight, we know one of them has failed. The instrument cross check is taught right from the Private Pilot License training. On the Private Pilot flight test, during the instrument flying portion, the examiner will actually have the pilot close their eyes while the examiner puts the aircraft into whats called an unusual attitude - an attitude the airplane will not normally be in, either extremely nose up or extremely nose down, with an element of bank in there as well. The student then has to quickly identify the problem and take the correct actions to recover before the airplane stalls or develops into a spiral. This is all with foggles on, glasses to simulated IMC conditions. The commercial flight test takes it one step further. The same thing is done, except both the Attidue and Heading Indicator are failed, and the pilot has to then recover using only the four remaining instruments.

Instrument failures can and do happen, and it would be foolish to place complete faith in one instrument. Instruments must always be verified by the information given on other instruments, and action taken immediately if a discrepancy is noticed.

Engine and nav instruments can also be cross checked to verify accuracy and identify problems, but I'll make that my next post if any of you found that interesting. Give me some feedback before I launch into another cockpit management related post.