Partial Panel Prognosis

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For most pilots the risk of instrument failure is remote. But when it happens, you have to be prepared.

When the AI rolls over and plays dead the pilot had better have a well defined plan B to execute.

Partial panel is often viewed as a loss of vacuum instruments—the gyros. In reality you have a partial panel any time you lose one or more of the required instruments regardless of how they are powered or even their particular function. Losing any instrument deprives you of information that may have a crucial impact on the safety of the flight. Few pilots think of the thermometer as an important instrument but when flying through clouds in the winter, it becomes vital. 

Pilots have become very dependent on the attitude indicator (AI)—probably too dependent. This instrument is critical to the safety of flight in IMC conditions. Pilots tend to forget it is not normally a primary instrument except when establishing a change in attitude for the aircraft. As a refresher, the primary pitch instruments are airspeed indicator and altimeter; primary bank are heading indicator (or mag compass) and turn and bank indicator and primary power are tach/mp gauge and airspeed indicator.

AIs can be vacuum or electrically powered. The emergence of glass panels has given the attitude indicator a new look and feel and (fortunately) a mandatory conventional AI as a backup. With today’s glass cockpits, a loss of the AI is not quite as serious as it is for pilots flying steam gauges.

With almost every cockpit sporting some form of GPS, the loss of a gyrocompass is likewise not as serious as in the past. There is less need to resort to timed turns or compensating for magnetic compass turning errors on the loss of a gyrocompass.

Older cockpits typically do not have all the bells and whistles of the newer cockpits. A vacuum pump failure may not elicit a warning light or a gyro gradually spinning down may not display an error flag. Loss of vacuum may be the more desired scenario since it tends to reveal itself faster than a gradually failing gyro instrument—the most insidious and dangerous type of instrument failure.

Stuff Happens

On a recent IFR trip, I had the displeasure of experiencing the loss of the AI. The symptoms were subtle. According to the AI the nose of my aircraft was slightly high. Cross checking with my other instruments I was flying straight and level. A quick check of the vacuum gauge showed plenty of suction. So, I adjusted the miniature airplane to the artificial horizon. This continued over the next 30 minutes. It was readily apparent something was wrong when I could no longer adjust the miniature airplane to the artificial horizon. During this time bank appeared to function normally on the AI. It wasn’t until the AI bottomed out that the gyro finally tilted over and died.

The IFR environment can become exceptionally challenging when one or more of the required instruments fails. Anticipation, preparation and training are important ingredients to a successful outcome.

Fortunately, I had several things going in my favor. First, I had an electrically powered HSI so I retained reliable directional information. While there was no indication of a vacuum pump failure gauges are known to read incorrectly so I was still glad I had the electric HSI. For those with vacuum powered gyrocompasses, crosschecking the gyrocompass frequently with the magnetic compass could help spot a pending failure sooner. While all gyrocompasses precess over time due to internal friction, experientially you have a feel for how rapidly this normally occurs for your particular airplane. 

Providentially, I had just completed an IPC, which included partial panel work. Having that recent experience may have saved my life. When is the last time you flew partial panel? Third, I was already alert that something was not quite right with the AI so I was mentally prepared for its inevitable demise. Fourth, and very importantly, my aircraft was trimmed for straight-and-level flight. I have the good fortune to own a plane that can be trimmed around all three axes. Being in trim makes partial panel flying easier. Lastly, the failure occurred in calm air.

When an instrument fails, maintain the cardinal rule of: aviate, navigate, and then communicate. As often suggested, cover the offending instruments. But how many pilots have something readily at hand to do that? Keep some instrument covers in the flight bag. A small pad of sticky notes works well, but stay focused on flying the aircraft. Since it was night, I loosened the bulb over the AI so while it was still visible it was not nearly as discernible as the other instruments.

Many applications that can run on a tablet or smart phone can provide a pseudo instrument panel. I would not want to trust my life to one of these without thoroughly testing how well it works beforehand. Unless the device is mounted in front of you, it could easily distract from the primary instrument scan and moving your head around in IMC is not a good idea—are you really ready to interpret an artificial AI mounted on the yoke tilted 30 degrees left or right?

The vanes in a vacuum pump eventually wear out and fail-usually without warning. Are you ready to fly IFR without the attitude and heading indicators?

One device you should take advantage of is the autopilot if so equipped. Even a simple wing-leveler can reduce the workload and anxiety. You do need to know how the system operates and is configured or you could easily make things worse. In-the-soup is no time to be learning aircraft systems.

In general, pilots are slow to recognize loss of instruments. How many have run out of gas due to a failed fuel gauge? Pilots also tend not to want to confess to ATC that they have a problem. ATC is there to help. Advise you are “no-gyro” rather than “loss of vacuum.” They should understand the former but not necessarily the latter. ATC is in the best position to get you to VMC conditions and can minimize future maneuvering required. They can also help get you to an airport that can offer an Approach Surveillance Radar (ASR) approach. If you have never done an ASR approach, you should experience it. If near a military base, the controllers typically appreciate the opportunity to practice these and some may have a Precision Approach Radar (PAR) approach, which are virtually non-existent at commercial airports. 

The key to survival in a partial panel situation is practice, preparedness, and knowing your aircraft.

Richard Lanning Ph.D. is a graduate of the U.S. Naval Academy and a pilot for more than 30 years. He is a FAASTeam member, an active CAP mission pilot, CFII and CFIG.

This article originally appeared in the August 2014 issue of IFR Refresher magazine.

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