Using GPS as Backup in IMC When Instruments Fail

A 31,000-hour 747 captain, lightplane owner and CFI discusses how an inexpensive handheld GPS receiver can serve as an effective backup in case of failure of gyro instruments, airspeed indicator, or even altimeter. The author contends that as GPS quickly becomes ubiquitous throughout the fleet, pilots should be trained to use it in equipment failure scenarios. He also discusses the use of the magnetic compass and ADF as backup instruments in IMC.


I am shocked how many pilots are unaware of howserious the situation is when all gyro-derived information is lost whileflying in IMC. If you are unable to determine whether your aircraft is turning,you will shortly end up in a fatal spiral. It doesn’t mater one whitwhether you are a brand new instrument pilot or whether you have 3,000 or4,000 hours of instrument time under your belt. You will be dead shortly.

 I have had many opportunities to demonstrate this to numerous student pilotsas well as to my high-time pilot friends. My demonstration begins by failingall the gyros including the turn coordinator (or turn-and-bank) while intrimmed level flight, with the pilot under the hood. In every case, the flightappears to continue normally for a short period of time. Inevitably one wingor the other will drop and a small turn will develop. Having no turn information,no corrective action is taken. As the turn develops, the nose begins to dropand the airspeed begins to increase. The pilot will usually recognize thosecues that something is happening and will increase the back pressure on theyoke in an effort to stop the increase in speed and loss of altitude.

Cessna SkymasterThis is the beginning of the end. As he pulls back on the yoke, this causesthe bank angle to increase. When the bank angle increases the airspeedaccelerates even faster. By now altitude is being lost at a greater and greaterrate and he begins to pull back on the yoke with an ever increasing force.This can generate enough load to fail some portion of the aircraft structureresulting in loss of control of the aircraft and certain death.

In order to maintain control of our aircraft while in IMC conditions, itis absolutely necessary to have turn information. We normally obtain thisinformation from the directional gyro with supporting information from theattitude indicator and turn coordinator instruments. But what if they fail?

Lets see if there is another source of information in our cockpits that canprovide turn information. There are in fact, several in most of today’saircraft.

GPS as a heading indicator

Like many of you, I was an early adopter of GPS navigation for aircraft.And it did not take me long to see the value of using the turn informationcreated by these systems as a substitute for turn information generated bygyros.

In fact, I just returned from a flight in my Cessna 182P that validated thetheory. I had a flight instructor as a safety pilot. While flying solelyby reference to instruments, the attitude indicator, heading indicator, turncoordinator and magnetic compass were all covered. My sole source of turninformation was the left/right “needle” of the Course Deviation Indicator(CDI) built into my Garmin GPS-150. And this proved sufficient information—alongwith the altimeter, airspeed and vertical speed indicators—to prevent theaircraft from entering deadly spiral.

In fact, I had the instructor simulate moderate turbulence with inputs fromhis yoke and was able to maintain straight and level flight. Like most GPSreceivers, my Garmin GPS-150 is equipped with the ability to update informationat the rate of once per second. This rapid update ability gave me the sensitivityneeded to prevent the spiral developing. In fact, I believe that any GPSreceiver could serve in this capacity, even the portable hand held unitsas long as it had the rapid update capability. Even a LORAN would probablywork, although not as well because it’s update rate is slower.

Recently, I purchased a GarminGPSMAP-195 handheld. This magnificent device combines an extraordinary12-channel GPS receiver with a remarkable high-resolution graphic display.And one of the “pages” that the device can display is an electronic HorizontalSituation Indicator (HSI) display which mimics the functionality of apanel-mounted HSI with uncanny precision…right down to its settableheading bug! I’m told that other modern GPS handhelds such as the II Morrow Precedus andLowrance AirMap have similarcapability.

GPS as a backup for pitot/static instruments

In the event of a pilot tube or airspeed indicator failure, the GPS groundspeedcould be used as a rough speed reference. And if the static system or altimeterfailed, the GPS altitude readout can provide an adequate altitude reference,give or take a couple of hundred feet.

Clearly, a modern GPS receiver is useful for more than its intended navigationpurpose and could serve a pilot well in the event almost any conceivableflight instrument failure. And now that GPS is found in almost every aircraft,pilots should be trained in using the GPS for this purpose.

Interestingly enough, there has been some experimental work done recentlywith GPS-based attitude indicators. If you mount a GPS antenna on each wingtipand program a GPS receiver to monitor both antennas and compare the “GPSaltitude” of the two wingtips, the GPS can display roll attitude quiteaccurately. It turns out that Selective Availability and other GPS errorscancel out, since you’re only interested in the difference betweenthe altitudes of the two wingtips, not the actual altitudes! So it’s entirelywithin the realm of possibility that GPS may replace all gyros in our panelsof the future…or that gyros may be considered backups for the GPS (insteadof the other way around).

The “whiskey compass” as a backup

The magnetic compass is another source of turn information that is littleused. The reason is that most pilots promptly forgot everything they learnedabout magnetic compass turning and acceleration errors about five minutesafter they passed their instrument checkride.

The most important thing to remember about the magnetic compass turning errorsis what the compass does when you’re on a southerly heading in the northernhemisphere: the compass leads any turn by approximately your latitude. (I.e.,if your present location is 30 degrees north latitude, the compass will leadyour turns by approximately 30 degrees.) The amount the compass leads yourturn is not nearly so important as the fact that it is indicating a turnin the correct direction.

In other words, the magnetic compass can and does provide correct turninformation while on a southerly heading. If you can manage to turn youraircraft to a southerly heading before all your gyro-derived informationis lost, you should be able to prevent the development of the deadly spiralby reference to the magnetic compass.

Whenever I demonstrate the ability to maintain control under the hood solelyby using the magnetic compass, I usually end up flying a series of shallowbanked turns to the left and to the right, but in general maintaining a southerlyheading. These are the result of using the compass to indicate when it isnecessary for me to input aileron control to stop any turn that develops.When the compass stops indicating a turn, I neutralize my aileron input.However, I have usually input more control than necessary to stop the turnand end up making a turn in the opposite direction.

I must give credit for my source of information about using the magneticcompass as a turn information source to a World War II B-17 pilot whose nameI have forgotten. He was a featured speaker at some long-ago luncheon andrelated how this technique got more than one battle damaged aircraft to safelydescend through IMC conditions to VFR conditions where they could then safelynavigate to their home bases.

To complete this discussion of the magnetic compass, you will recall themagnetic compass will lag any turn while on a northerly heading. In factit will not indicate any turn at all while on a northerly heading until aheading change of approximately of 30 degrees has occurred if the turn isa very small bank angle.

The magnetic compass will, while on an easterly or westerly heading, incorrectlyindicate a turn due to acceleration or deceleration. If you are on a westerlyor easterly heading, acceleration will indicate a turn towards the north.Conversely, if you are on an easterly or westerly heading, deceleration willcause the magnetic compass to indicate a turn towards the south.

It is important to note that there is no acceleration or deceleration turningerror while flying either a northerly or southerly heading. While flyinga southerly heading (in the Northern Hemisphere) the magnetic compass willactually lead any turn and indicate the turn is occurring in the properdirection. In fact, you might consider the compass as extremelybank-angle-sensitive in the proper direction while on a southerly heading.

To complete this discussion of alternative turn information sources foundin today’s cockpits, I would point out the Automatic Direction Finder(ADF) found in many aircraft is an excellent source of heading informationin the event of loss of all gyros. Simply tune the radio to some facilitymore-or-less ahead of the aircraft and use the needle to provide your neededturn information.