Aircraft Instrument Systems: A Brief Guide

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Do you know what instruments you can rely on to provide accurate information when the unexpected happens? A&P Mike Berry discloses what you absolutely need to know about your aircraft instruments. 

Aircraft instruments have been a part of aviation since the first flight of the Wright Flyer, which was equipped with a stopwatch, an anemometer (to measure wind speed) and a tachometer. 

With the increase of flight activity in the early years of aviation, aircraft instruments were invented to provide necessary information to pilots for precise control and navigation of their aircraft. 

As a pilot and aircraft owner, it is important to understand not only how aircraft instruments work, but also to be knowledgeable of the systems that they interface with. 

The maintenance and care of an aircraft, including its systems and required inspections, are tasks that the aircraft owner is responsible for—and they are not easy. 

In this article I will give some insight into instrument repair and replacement options as well as the maintenance and repair of systems that drive these systems. 

The basics, and some important questions

All modern aircraft, whether the aircraft has digital or analog instruments, share the same basic pitot and static systems. These systems deliver a very slight pressure to the instruments that they serve, and instrument accuracy is impacted by even slight variations. Leaks, disturbed air or even partial blockage in the lines serving instruments such as the altimeter, airspeed, and vertical speed indicators will certainly affect accuracy. 

There are other systems that are electrical or mechanical in nature and for the most part are self-energized such as the tachometer, oil pressure and oil temperature gauges. While the latest models of aircraft have electrically powered instrumentation, the majority of General Aviation aircraft still retain the self-powered instruments as a matter of reliability and economics. 

It is important as an aircraft owner and pilot to know the basics. In case of a total electrical failure, what instruments can you rely on to continue to provide you with accurate information? For example, fuel quantity gauges on most aircraft require electrical power and will not be reliable with the electrical system shut down. 

Consider the vacuum system that powers most General Aviation gyroscopic instruments such as an artificial horizon (AH) and gyroscopic heading indicator (DG). When a vacuum pump fails, what instruments can you rely on? 

Will your autopilot work? Will a failure of one vacuum instrument cause the other vacuum instruments to fail shortly thereafter? How about the old turn-and-bank or more modern turn coordinator instrument; how are they powered? 

Turn coordinators are electrically powered; a turn-and-bank is powered by vacuum from the engine-driven vacuum pump. 

The most important aspect of any gyroscopic instrument is that a failure may not be immediately noticeable unless the aircraft is equipped with a warning system. 

In the case of a failed pump supplying vacuum pressure to gyroscopic instruments, the instruments will decelerate and become inaccurate over a minute or two, not in mere seconds. This inaccuracy over time can cause a pilot to lose control of the aircraft by following a slowly dying gyro into the ground. Several fatal accidents have occurred over the years for just this reason, and a low vacuum warning can be a lifesaver. 

This panel of this Skylane is equipped with a low vacuum warning and voltmeter.
The rules concerning aircraft instruments

FAR 91.205 specifies required instruments for VFR flight for the most basic aircraft. These consist of an airspeed indicator, altimeter, compass, fuel quantity, oil temperature and pressure, and tachometer. These instruments must be operational for an aircraft to be considered airworthy. 

There may be additional required instruments associated with the specific operations of the aircraft (such as instrument flight rules) and even some instrument requirements specified by ADs, Type Certificate Data Sheets, flight manuals or supplements and STCs. 

It is up to the pilot in command to determine that the required instruments are operational before flight, and that the instruments are certified for the operation intended. While some instruments may legally be inoperative, consideration must be given as to how an inoperative instrument will affect the operation of the aircraft. 

Additional rules concerning aircraft instruments according to 14CFR 65.81, General Privileges and Limitations, are that “… a certificated mechanic… is not permitted to… accomplish any repair to or alteration of instruments. These activities are reserved for certificated repairmen at an authorized repair station.” 

This means that anything other than an external adjustment of an instrument—including installing a compass repair kit—is not authorized. 

Static systems test and inspection for IFR flight is required by FAR 91.411 and must be accomplished every 24 months or “Except for the use of system drain and alternate static pressure valves, following any opening and closing of the static pressure system, that system has been tested and inspected and found to comply with paragraph (a), appendix E, of part 43 of this chapter; and (3) Following installation or maintenance on the automatic pressure altitude reporting system of the ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and found to comply with paragraph (c), appendix E, of part 43 of this chapter.” 

This means a certificated mechanic with the proper test equipment can certify only the static system (checking for leaks) and not the altimeter or transponder portion which is referenced in FAR 43 appendix E.

EGT instrumentation must use the correct color-coded wiring and may not be spliced, repaired, or in any way modified from the original configuration, including length.
How instruments operate, and why they fail

Traditional (steam gauge) aircraft instruments can be grouped according 
to their operating systems. 

Instruments such as these are often sold on eBay in unknown condition, and may not operate or cannot be repaired. If you are buying an instrument at a flea market or on eBay, you should not only ensure that it can be repaired and certified, but make certain that it is appropriate to your aircraft.
Pressure instruments

Pressure flight instruments operate off of the static and pitot system, are self-powered and extremely sensitive diaphragm-type instruments relying only on variations in pressure to operate. These pressure variations are transmitted mechanically by gears and a jeweled movement as a result of the extension and retractions of the diaphragm. 

As with anything mechanical, age takes its toll on the accuracy of pressure instruments such as the airspeed, altimeter and vertical speed indicator (VSI). These instruments are affected by moisture as well as dust and dirt, and should be kept clean. 

Cloudy or dusty-looking instruments may mean that the system is contaminated and the static system must be purged of moisture or dust and the instruments promptly repaired or replaced. Leakage sometimes occurs between the instrument glass and outer case as well as inside system fittings. Sealants become inflexible over time and lose their ability to keep the system closed. Leakage must not be tolerated, as the accuracy of all the instruments in that system is compromised. 

Aircraft instruments are delicate and require special equipment and training to be successfully repaired.

Instrument repair shops operate as FAA approved repair stations and while all instrument shops adhere to the same FAA rules, some shops may be authorized to do specific repairs while others may not.
Vacuum instruments

Vacuum operated (gyroscopic) instruments have been very reliable over the years, with very few actual failures of the instruments themselves; however, these instruments are subject to malfunction when an aircraft vacuum system fails. 

Vacuum system failures can be prevented with proper care and maintenance (or replacement of components) as specified by the aircraft manufacturer. 

One often-overlooked procedure is to check the vacuum gauge reading in your aircraft against a calibrated gauge. This ensures that the actual vacuum/pressure is set correctly, as over-pressure or under-pressure compromises accuracy, increases wear and creates an opportunity for failure of instruments or the entire system. Another often-overlooked but recommended procedure is to replace both pressure and vacuum filters on an annual basis. 

When replacing a vacuum pump due to a failure, ensure that all hoses, filters and fittings are checked for contamination from foreign material as not only is the newly-installed pump at risk of failure, the instruments may also fail due to foreign material contamination. 

Vacuum instruments are mechanical devices that operate with a gyro spinning at high speed powered by jets of vacuum or pressure impacting on small cups machined into the gyro rotor. The precision-balanced rotor is suspended by a shaft and supported by tiny bearings which are lubricated when the instrument is assembled. There is no provision for lubrication other than when the unit is disassembled during maintenance or repair. 

Gyros rarely fail without some type of warning which may be indicated by excessive drift or precession, noisy or erratic operation. Inactivity really takes its toll on these instruments as the lubrication that is on the tiny bearings tends to drip or wick away from the actual bearing surfaces when the instrument is at rest for long periods of time. 

Inaccurate readings of an airspeed indicator can have a definite impact on performance and overall safety. This example is unairworthy.
Electric instruments

Electrically powered instruments can be of several different configurations, from a simple fuel quantity sender or flap position sender (variable resistor) and indicator, to an electrical tachometer powered by a small generator (though a flexible mechanical cable between the engine and the gauge in the instrument panel is more common). 

EGT and CHT gauges are usually self-powered relying on dissimilar metals in the sender or sensor to generate an electrical signal directly to the gauge on the instrument panel. The color coding of the wires is important as senders with different color coding than the instrument will not be compatible. 

Sending units and wiring for CHT/EGT gauges must not be repaired, spliced, or in any way modified from the original configuration—including length. If it’s broken, replace it. 

Anything electrical is subject to the effects of vibration, corrosion and broken (open) connections; remember this in your troubleshooting routine. 

Also significant in any electrical instrument installation is that individual components of a system are in most cases not interchangeable. For example, a Rochester brand gauge must be connected to a specific type of sender unit intended for use with the Rochester gauge; a Stewart Warner brand sender may not work properly with a Rochester gauge. 

Mistakes can be costly; check the schematic diagrams for the proper wiring, refer to the parts manual for the compatible component, and physically check that the item is what is actually installed in the aircraft you are working on. 

Electrical components do wear out and/or deteriorate over time and malfunction, even if the item is rarely used. Good preventive maintenance practices—such as keeping moisture off of connections, proper routing and attachment of wiring, and reducing airframe vibration—can go a long way in avoiding premature instrument and electrical failures. 

The compass is a required instrument and must have a correction card. According to 14CFR 65.81, General Privileges and Limitations, repair to or alteration of instruments are reserved for certificated repairmen at an authorized repair station.
Repair options

Finding a shop that will work on older instruments is becoming difficult if not impossible, and often owner-pilots are left with no option but to replace an instrument. 

The rules of requiring approved technical data covering repairs and overhauls, approved parts sourcing and proper repair and test equipment are alive and well in the aircraft instrument arena. For this reason, many instruments that were original equipment on General Aviation aircraft 30 to 50 years ago are no longer supported and are not repairable. 

Fuel gauges must be operational, and most can be repaired.
Authorized shops

Instrument repair shops operate as FAA approved repair stations and while all instrument shops adhere to the same FAA rules, some shops may be authorized to do repairs while others may not. 

Do some checking around to see if 
you can find a shop that does repair older instruments. There are some, such as Air Parts of Lock Haven, that specialize in older aircraft instruments and in fact have repair station authority to do extensive repairs. 

Air Parts of Lock Haven also has access to repair parts sources that other shops may not have. Air Parts of Lock Haven repairs older instruments and can also duplicate original instrument dials and faces. 

Many airspeed indicators are aircraft-specific; know your requirements before purchase.
Radioactive components

Many instruments that were supplied as original equipment in the 1940s and 1950s and even into the 1960s came with luminous dials and markings which happen to be radioactive and are now considered hazardous material. 

If you have one of these instruments, it must be shipped as hazardous material with all the markings, shipping labels and details that pertain to hazardous material. Few shops are equipped to handle this material and will refuse the shipment. 

At last check, Air Parts of Lock Haven can receive these instruments and has authority to handle the material, but the instrument will not be returned with the radioactive dials.

Fuel quantity senders wear out and need to be serviced by an approved facility. 
General shipping information

Any instrument that requires shipment to a repair shop must be packaged properly—as if you were shipping eggs—and the package should be marked as fragile and insured. 

It would be prudent to call the instrument shop you are shipping to and ask for a carton to ship an instrument in and wait a few days for the container to arrive rather than risk damage to the instrument in shipping. 

Unfortunately, shipping companies can and do damage aviation material—and an insurance adjuster’s value of the instrument may be much less than what a functional instrument may actually cost. 

Post World War II instruments such as the ones installed in this early model Cessna 172 may have luminous dials that contain radium. Few repair shops are equipped to handle this material and may refuse the shipment.
Buyer beware

A word to the wise: if you are buying an instrument at a flea market or on eBay, not only ensure that it can be repaired and certified, but make certain that it is appropriate to your aircraft.

Markings on a replacement airspeed indicator, for example, must be specific to the make and model of aircraft, and the details may be found in an official flight manual, TCDS, STC-related flight manual supplement, or even AD notes or Service Bulletins. 

If you send in an airspeed indicator with a specific aircraft manufacturers’ part number, what you will get back is a repaired or a replacement airspeed indicator that will have the markings appropriate to that particular part number—which may or may not have the correct markings for your aircraft. There is no choice here as to changing the markings, and adding or deleting marks is not permitted by the FAA. 

The importance of accuracy for performance

The performance listed for your aircraft was obtained when the aircraft, engine and propeller were new, and the aircraft was rigged properly, loaded to the most favorable center of gravity location and flown by a test pilot under optimum atmospheric conditions with accurate instrumentation. While it is possible to duplicate the published performance numbers with an older aircraft, everything must be nearly perfect to do so, and accurate instrumentation plays a big role. 

Although digital instrumentation is replacing analog instruments and equipment, much of the instrumentation still relies on precise pitot or static system pressure which is then delivered to the computer or other device to indicate airspeed, altitude or vertical speed. 

So, unless you have precise pressure, the 78 knots indicated you are using to achieve best rate of climb may not be exactly 78 knots. In addition, mechanical tachometers, whether due to age or inactivity, have a history of being inaccurate. 

Inaccurate readings from just these two instruments—airspeed and tachometer—can have a very definite impact on performance and overall safety, as the aircraft will not achieve published performance numbers. 

Most, if not all, aircraft maintenance shops have tachometer checking equipment and the calibrated tachometer checker should be used to compare required static rpm listed on the TCDS to the aircraft’s actual full-throttle revolutions per minute. An aircraft tachometer can easily differ from the published requirements by 100 rpm or more and some aircraft are rejected during annual inspection because of this. 
Practical application

Any aircraft owner knows that aircraft are expensive to maintain and there is no indication that costs will come down. Aircraft instruments are no exception; however, there are some economical ways to determine if you do have instruments that are in need of repair or replacement. 

System leaks

Static system leaks, for example, can often be discovered by some simple tests. Does the VSI, airspeed or altimeter needle move when a door or window is closed or opened while on the ground with the engine not running? When you open the cabin heat valve or a window in flight, do any of the three instruments just mentioned move abruptly? 

Unusual temporary indications may indicate a leaking system component such as an alternate static port, leaking instrument glass or a broken or cracked moisture trap. 

Altitude discrepancies

Also consider the effect of modifications to your aircraft, as these may impact the static system and overall instrument accuracy. An example of this was an aircraft that was modified with a cargo pod and several electronic sensors for aerial survey operations. 

When the modifications were completed, the aircraft was test flown and at higher altitudes (in the teens). An instrument accuracy check revealed a 900-foot error in the actual altitude versus indicated altitude. 

Errors such as this are rare, but can happen, so be especially vigilant when multiple modifications are made to an aircraft. The possible combined effect these may have on actual versus indicated altitude is worth examining. 

An unofficial altitude comparison can be made between a GPS unit’s derived altitude and the indicated altitude while in flight. Large errors—such as a difference of a few hundred feet or more—should be cause for further investigation into pressure instrument (altimeter) and static system accuracy. 

Electrical fuel quantity

Fuel gauges are another set of instruments that are known to be inaccurate, yet pilots rely on them. A typical electrical fuel quantity system on General Aviation aircraft consists of three parts: the sending unit (using a variable resistor attached to a mechanical arm/float), electrical wiring, and an indicator in the cockpit. 

The sending unit attached to the fuel tank can fail mechanically or electrically, or provide inaccurate readings as both parts can wear or age. The float can absorb fuel and partially sink, providing an erroneous indication. Electrical wiring can become corroded or disconnected, and if a complete circuit is not maintained, may indicate full all the time (or empty all the time). 

Some basic troubleshooting by a technician with a voltmeter and schematic can determine the offending component fairly quickly—especially when the plane is opened up for annual inspection. 

A fuel gauge indicating the quantity of fuel in each tank is one of the required instruments according to FAR 91.205, and most (if not all) components—even on the most ancient aircraft—can be repaired or replaced to make the system work properly. 

Be proactive

As a pilot or aircraft owner/operator it is very important that you properly maintain aircraft instruments and associated systems as well as seek repairs or replacement at the first sign of any deficiency. Operating an aircraft with a faulty or inoperative instrument can have serious consequences. 

Maintenance personnel conducting an annual or 100-hour inspection should not return an aircraft to service, and pilots should not conduct flights with inoperative instrumentation or equipment required by FAR 91.205. 

Michael Berry, a former aircraft repair shop owner, is a multi-engine rated ATP (757/727). In addition, he’s a turbo jet flight engineer, an A&P/IA mechanic, airplane owner and 121 air carrier captain. Berry has 15,000-plus pilot hours. Send questions or comments to .

Resources

Air Parts of Lock Haven 
airpartsoflockhaven.com

Further reading
FAR 91.205
“Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements”

FAR 91.411
“Altimeter system and altitude reporting equipment tests and inspections”

Appendix E to FAR Part 43 
“Altimeter System Test and Inspection”

14CFR 65.81 
“General Privileges and Limitations”

All of the above documents are available at the FAA website: 
rgl.faa.gov