Even some VFR pilots — to keep the aircraft upright in marginal conditions (hazy days of summer included) often use the Attitude Direction Indicator (ADI) — but even good instruments fail. A simple device, it depicts the aircraft’s attitude using a horizon bar and a ball. Half of the ball is colored blue (in most cases) to match the sky, while the other half is black or brown to indicate it represents the ground that we are flying over.
WHETHER YOU ARE VFR OR IFR, many pilots use the ADI to keep their wings level and to stay on track. I have. While this isn’t illegal, under marginal conditions, it does open you up to an area of risk — that is the risk of an instrument failure.
WHEN ADIs FAIL, ALL BETS ARE OFF. We’ve seen failures where the ADI got “lazy,” and other failures where it just got stuck. The problem is that you won’t know what type of failure you have until some time after you notice a problem, and by that time, you may already be in trouble. If that happens, you could get yourself into an unusual attitude, and in doing so, wind up pushing your aircraft beyond the structural envelope for which it was designed.
INSTRUMENT PILOTS ARE TRAINED TO CROSSCHECK THEIR INSTRUMENTS. This is how they keep one bad instrument from sending them into a death spiral — they are constantly checking and rechecking the instruments. This is called the “instrument scan,” and is one of the things that allows the instrument pilot to fly safely in the clouds. The well-trained and practiced instrument pilot is not dependent on a single instrument, so a single instrument failure is often more of an inconvenience than anything else. Sometimes, however, the consequences are far more severe.
ADI DANGER SIGNS
- Slow to Erect — This is a sign of bearing problems in the ADI and, no, it is not solved with Viagra. If it takes more than about 30 seconds in a vacuum pump equipped plane, start to get nervous.
- Doesn’t Settle When Shutdown — Does your ADI stay in the “level” configuration when you shut down your engine? If so, the bearings are already shot, and the ADI needs to be replaced as soon as possible.
- Lazy In Flight — If your ADI is slow to react in flight, despite having good vacuum or pressure on your instrument system, this is another sign that your ADI is getting ready to give up.
- Loud Noise on Shutdown — If your ADI makes squealing noises, or grinding noises on shutdown… well, that isn’t a good thing.
READ AND HEED THESE SYMPTOMS — Any one of these symptoms, should be a cue to get your ADI into the shop for an immediate checkup. Most local avionics shop will be able to connect their own shop vacuum supply and place the ADI on a tilt table, to test how the device is working. If it isn’t working well, this system will help a trained eye quickly find the problem.
IF YOU SEE MORE THAN ONE OF THESE SYMPTOMS
- Do not fly in instrument conditions until the ADI has been checked out and either receives a clean bill of health (unlikely) or is overhauled or replaced.
- No marginal VFR flights should be attempted.
The problem is simple — the ADI is trying to send you a message that it won’t be around much longer.
While nobody can predict the exact time that the instrument will flip out and quit, Murphy’s Law dictates that it will usually happen at the least opportune moment. If the weather is poor, the ADI is not in good health, and you fly anyway… You’re not taking a chance on the instrument; you’re taking a chance with your life. Have the instrument checked out before you intend to use it.
BOTTOM LINE: Fix it before it becomes a problem. Proactive maintenance on the ground does not compare with reactive maintenance in the air. Maintenance can be inconvenient and expensive, but it’s still much better than the alternatives.
