Murphy had a great gift for simplifying technical problems in an unforgettable way. The first law: “If a part can be installed wrong, it will be.” Here is Murphy’s law as it applies to instrument flying: “If an instrument can fail, it will.”
Here, in summary form, is a list of instruments and problems to prepare for.
Attitude indicator. This instrument is driven by gyros powered by the vacuum system. The indicator fails when there is a failure in the vacuum system, the result, usually, of an engine-driven vacuum pump failure. Failure of the attitude indicator shows up gradually. The instrument doesn’t just roll over and die but begins to drift off slowly at first as its gyros wind down. For a good idea of what this looks like, watch the way an attitude indicator behaves after engine shutdown at the end of a flight.
If you have trouble maintaining straight and level, stabilized climbs and descents, or smooth standard rate turns, suspect failure of the attitude indicator. Cover it up to prevent scan distraction and switch to partial panel operation, which is explained in Chapter 12.
The attitude indicator might also show erroneous information if it has been set in-correctly. There is only one situation in which the attitude indicator can be set in-correctly.
And that is straight and level unaccelerated flight. That is the only time the miniature air-plane can be accurately matched to the horizon line.
97 If you reset the attitude indicator on the flight line, it will be incorrect due to the nose-up attitude of the parked airplane. Other errors will occur if a reset is attempted in turns, climbs, and descents.
Heading indicator. Like the attitude indicator, the heading indicator is driven by the vacuum system. It will fail if the vacuum system fails, and it can also fail when the vac-uum system is operating normally. Like the attitude indicator, a failure of the heading in-dicator is rarely dramatic. If you suspect a failure, cover the instrument and switch to partial panel operation.
Make sure that the heading indicator shows the same heading as the magnetic com-pass. Students make three common mistakes when setting the heading indicator.
First, they fail to reset the heading indicator when lined up on the runway centerline for takeoff. This is the best time to reset a heading indicator because you know the run-way heading. If it’s an instrument runrun-way, the heading is often shown to the last degree on an approach chart instead of rounded to the nearest 10 interval.
Second, because of precession the heading indicator slowly drifts off. It should be re-set every 15 minutes, or after maneuvers that involve a lot of turns in a short time, such as holding patterns.
Third, the heading indicator cannot be reset accurately unless the airplane is in straight and level, unaccelerated flight. The magnetic compass is the culprit here.
It is accurate only when stabilized in straight and level, unaccelerated flight. (Chapter 12 details how to cope with magnetic compass errors.) Meanwhile, resist the tempta-tion to reset the heading indicator in a turn. I have seen students make errors of as much as 30 while trying to match heading indicator and magnetic compass in a turn.
Magnetic compass. Chapter 12 has more about the magnetic compass, but let’s re-view a few points learned in VFR training:
• In calculating headings, account for variation due to the earth’s magnetic field, and deviation due to magnetic influences on a specific compass because of its location in the airplane.
• Turning toward the north, the compass lags behind the turn due to dip error, turning toward the south, the compass leads the turn.
• On easterly and westerly headings, acceleration produces an indication to the north, deceleration produces an indication to the south. Remember ANDS (Acceleration North Deceleration South).
Altimeter. The altimeter will read erroneously if not set to the correct barometric pressure at all times. It will also read erroneously if the static port is clogged. Insects, ice, and dirt can clog a static port. The problem becomes apparent when airborne and the al-timeter needles don’t move.
An alternate static source aboard an airplane can restore the altimeter to normal op-eration if the static port has become clogged: however, the altimeter will read higher than normal.
If you do not have an alternate static source aboard, create one by breaking the glass face of the VSI. This vents the pitot-static system to the cabin, the same as an alternate static source. Again, the altimeter will read higher than normal. (Breaking the glass face of the VSI usually damages the needle and renders the instrument inoperative.)
Airspeed indicator. A blockage of the pitot tube will render the airspeed indicator useless. As is the case with the altimeter, you won’t know this until airborne. Pitot heat will prevent ice from clogging the pitot tube. That’s why I recommend turning on the pitot heat before takeoff on every instrument flight. When ice has clogged the tube, pitot heat might melt it too slowly. (This is developing good habit patterns.)
Insects love to nest in the pitot tube; use the pitot tube cover. Insects can clog those tiny air passages in just a few minutes.
Blockage of the static source also causes erroneous airspeed readings. An alternate static source will produce an indicated airspeed a knot or two faster than normal.
Vertical speed indicator (VSI). The most important thing to remember about the VSI is that it only gives an accurate reading when the needle has been stabilized for 7 seconds or longer. If the needle is moving, forget it.
Another quirk of the VSI is that when you first raise the nose of the airplane to begin a climb, the VSI needle initially shows a descent. The reverse is true in the beginning of a descent when the needle will momentarily show a climb.
The needle of the VSI should point to zero when the aircraft is sitting on the ground.
If not, the needle can usually be zeroed by turning a small screw at the lower left corner of the instrument case. If this adjustment cannot be made, add or subtract the error for an accurate reading in flight.
Turn coordinator. The turn coordinator is powered by electricity; it will continue to operate even if there is a failure of the vacuum-powered attitude indicator and heading in-dicator. If the turn coordinator fails, the needle won’t move. It will remain fixed in an up-right position. That’s why it’s important to check the movement of the turn coordinator while taxiing out. It’s pretty rare, but I have also seen the ball of the turn coordinator get stuck in the tube.
Fuel gauges. Here’s another Murphy’s law: “On land, air, and sea, the second half of the tank always empties faster than the first half.”
Never trust any fuel gauges. There is no way to judge how accurate they are and most of them are fairly crude. Always note takeoff time and the time en route from each major position fix, then calculate fuel consumption based upon airplane performance figures.
Ask “What if my fuel gauges failed completely? Am I keeping track of fuel calculations well enough independently of the gauges to know exactly how much more flying time I have left?” (It is time in your tanks.)
Oil pressure and temperature gauges. Engine instruments should be scanned ev-ery few minutes. The main concerns are low oil pressure and rising oil temperature.
When these symptoms appear, a serious problem is developing in the engine oil system.
Land the airplane as soon as possible. Don’t stop to think about whether or not the gauges are functioning properly.
99 Low oil pressure with no rising oil temperature indicates either an instrument error or an incorrectly set pressure relief valve. Keep an eye on this situation. As long as the in-dications remain stable, the flight can be continued. But if the oil pressure drops and the oil temperature rises, land.
High oil pressure with normal oil temperature usually means that the pressure relief valve has been set incorrectly. As long as engine indications remain normal there is no reason to discontinue the flight.