Liar, Liar, Pants on Fire

Like anything else, your instruments occasionally will betray your trust in them


Theres an old saying in aviation that youll hear throughout your flying career: Trust your instruments.

Anyone who has had instrument training has been beaten over the head by the instructor. The strategy is necessary in order to overcome your proprioceptive sensors – the so-called seat of the pants sensations – that you learned to rely on when flying VFR.

Yet, what may not have been emphasized is that your instruments can and occasionally will lie to you. Therefore a basic function of cockpit resource management requires a continuous cross-check of the engine, flight and navigation instruments. Its akin to the old saw, just because youre paranoid, dont think someones not out to get you. Sooner or later an erroneous instrument indication will rise up with the potential to get you into serious trouble.

During the Korean War I had an electrical slave gyro-compass error that almost took me to China. As a brand new military pilot my training had been the best, and always believe your instruments had been burned into each of us. But fortunately the habits formed while flying a Piper Cub in college caused me to look up casually at the magnetic compass and see that it was 180 degrees different from the slave gyro heading.

At first it was confusing. In the back of my mind I kept hearing always believe your instruments. But in this case, two instruments I had always trusted disagreed completely. Naturally I followed the mag compass and was able to reach home base safely. While this was many years ago, slave gyro errors can happen in the newest and fanciest airplane on the flightline.

Magnetic Compass
In a more recent occurrence, during a series of practice ILS approaches in an aero clubs Cessna 182, I was consistently unable to get aligned with the localizer course centerline. While in this instance the weather was clear, during practice the previous week there had been a 400 foot broken ceiling and 2 miles visibility. After a couple of semi-sarcastic comments from the otherwise tolerant ATC controller, I told him my DG was precessing excessively and terminated the flight.

When I wrote in the maintenance records that the directional gyro had excessive precession, two of my fellow members cornered me and said Oh its not the DG, its the mag compass. We had the airplane in Idaho last week and it was 20-30 degrees off on some headings. Just watch the section lines and youll not have a problem.

The irony is that one of them was the club maintenance officer, and this was his apparent resolution of the problem. They had attempted already to replenish the fluid in the compass, which was somewhat low, but the error persisted.

There were several problems with this reaction. They were trying to save $50 by fixing the compass themselves, but only a certified instrument technician can work on instruments. Telling fellow members to watch the section lines as a means of navigation is almost laughable until you think of the consequences of getting lost over the desert southwest.

The magnetic compass is an airworthiness item. It is required equipment even for basic VFR flight. In short, you simply cannot legally fly an airplane without a functional and accurate magnetic compass. And there must be a compass card showing deviation on different headings.

Legal hair-splitting aside, think of the hazard to a pilot flying IFR in mountainous terrain with a 20-30 degree magnetic compass error. On any VFR flight navigation would be nearly impossible with that much compass error.

There are other things that can cause severe mag compass errors. One of the most insidious is a flashlight left on the instrument shroud in case its needed on a night flight. This can cause gross errors that make the compass unusable. A camera or a hand-held radio will do the same thing.

But there are other sources of compass error as well. In some models, activating the pitot heat causes mag compass errors. In my previous airplane, a Piper Arrow III, activating pitot heat caused the mag compass to deviate eight to 10 degrees. Try flying a VOR or ILS approach with that much error. To set the directional gyro for an IMC approach, you must first turn off pitot heat; set the DG, then re-select the pitot heat.

For those lucky enough to fly fancier equipment, there are the subtle HSI or slave gyro errors that can cause serious problems.

Once I was cruising at FL 370 on J-85, generally westbound, in a Sabre 60. We were exactly on course and in Navloc (automatic navigation) on the flight director. Then ATC asked if we were not cleared J-85; to which the co-pilot responded Affirmative. The controller then said, Well, I show you 10 miles south of course. Turn right 20 degrees and intercept the airway.

I was confused. After a quick check of the chart to confirm the track, I looked at the co-pilots VOR. His CDI too showed us on course. Then I checked the inverter voltage. It read the required 115VAC, but the cycles were very near the minimum of 390.

On a hunch I switched the inverter manually to number two. Instantly my CDI went full scale off course. The HSI too changed heading about 30 degrees. A check of the voltage showed 115 VAC with a normal 400 cycles.

The consequences of the error could have been severe if we had been in mountainous terrain at low altitude and in a non-radar environment, e.g. parts of Montana, Wyoming, Utah or Alaska.

The AC-powered HSI can play other tricks on you too. Consequently you must frequently cross-check its indications with the magnetic compass. The most prevalent error I have encountered has been during line-up for departure. As the tower says cleared for take off you look down and find the slave gyro or HSI 30, 60, 90 or 180 degrees off the runway heading. But without the proper habit patterns you may not notice it.

To hedge against such errors it is proper resource management to set your heading bug on the surveyed runway heading for the departure runway. Best time to do this is just after getting the ATIS. Then you can ensure that the gyro slaves to the correct heading.

If the taxiway parallels the runway, then while taxiing out, you will note that the heading bug is on the index mark at the bottom of the HSI. Once you line up for takeoff, the heading bug should be on the index mark at the top of the HSI. This confirms that you are aligned on the correct runway and that your gyro is properly slaved.

I once flew as co-pilot on a Sabreliner 60 for a captain who, upon lineup for departure, insisted on setting the heading bug to the first assigned heading after take off. However mine was set for the surveyed runway heading. We were departing La Guardia Airport on runway 13. It was IFR and raining, with a low ceiling – which exacerbated the usual rush-to-get-going situation. I had been busy with the checklist and of course the radio and failed to notice the heading bug. At around 100 knots I looked down and saw it skewed 30 degrees to the right of the runway. The Captains heading too was 30 degrees off. The HSIs had gone out of slave unnoticed as we took the runway.

Needless to say you dont want to be 30 degrees off your heading in IMC conditions in the New York area. I called it and immediately began fast slaving my instrument. The captain leaned over and did the same. Mercifully, by the time we were airborne and climbing, the gyros were re-aligned.

There is yet another reason to align the heading bug with the departure runway, particularly at night or in precipitation or other low-visibility conditions.

As you gather speed during the ground-roll, it is common to have rain or snow impinging on the windscreen, which can reduce your forward visibility to almost nothing. Thus whether you abort or continue you need some reference to keep you aligned straight down the runway.

In this case the heading bug is literally a lifesaver, especially with coordinated use of flight director V-bars or cross-pointers. Otherwise you may hit a snow bank along the edge, or run off the side into the mud and ultimately collide with something at high speed. Without fail, this type of accident happens every year.

Navigation Errors
GPS is fast becoming our primary means of navigation. For aircraft that do not have a receiver installed, a clip-on or handheld unit has become quite popular. You see them in everything from Cubs to Citations. These are useful little boxes, but legal as primary navigation tools only for VFR flying, although we all know they are used in IFR conditions too.

You may fly for years with no problem, but the one time a satellite is down and you get faulty information could find you lost or up against the side of a mountain.

Using unapproved devices for IFR navigation can lead to unexpected results. More than two decades ago a Sabreliner 40 was returning from Europe with a load of passengers. It was late at night and the captain was navigating using a then-new and as-yet unapproved Global 200 VLF system. Like GPS, VLF had proved incredibly accurate and was gaining popularity quite rapidly. However, the sticky wicket was that the very low frequency signals were from U.S. Navy submarine communication stations, located at six points around the world. It was not an approved means of air navigation, but when everything was working it was exceedingly accurate.

As with modern GPS, this VLF system needed the signals from at least three or four stations to triangulate the airplanes position. Unfortunately on this night one of the required stations shut down for scheduled maintenance. Although it was scheduled maintenance there was no requirement for a NOTAM since it was not an approved navigation system.

Thus on a dark, moonless night the Sabreliner missed Goose Bay by a wide margin and ultimately crash-landed on what the pilot thought was a frozen lake. All aboard were killed.

Likewise, using a non-approved GPS as a sole source for IFR navigation or an instrument approach in IMC is unwise. Even with an IFR-approved unit, the pilot must monitor the normal ground stations, VOR or NDB, unless he has receiver autonomous integrity monitoring (RAIM) to verify signal integrity. Portable units do not have that capability. As the AIM emphasizes, without RAIM capability, the pilot has no assurance of the accuracy of the GPS position.

With a RAIM-equipped GPS, the pilot is not required to concurrently monitor the ground stations. However if one of the RAIM required satellites becomes corrupted or unavailable, then the pilot must actively cross check the airplanes position with the usual navigation aids. Here again, this is to guard against erroneous heading information.

In addition, remember that the GPS-derived altitude is not reliable. Because of the way GPS calculates altitude, the vertical error is too large even for backup altitude information in aircraft.

The nondirectional radio beacon (NDB) is a very untrustworthy means of navigating, particularly IFR. First of all, it is rare to find one that works properly even in good weather. When the weather is bad the NDB becomes even more unreliable, since precipitation and atmospheric static causes the ADF needle to wander. Then too, after sunset, there is night effect, which causes the ADF to home to erroneous signals from most any direction.

On occasion, while still 50 miles out, you may be homing with the ADF needle pointing directly toward what you believe to be the outer compass locator. But it could be pointing to another station, or tracking some type of interference.

I once saw a classic example of this on an instrument check ride. The pilot had misdialed the outer compass locator frequency to 210 instead of 310. By coincidence the 210 frequency was active at an airfield some 75 nm distant. And he committed the unpardonable sin of not listening to the station identifier. Fortunately it was a clear day, because we traveled halfway across Ohio awaiting station passage.

The compass locator is normally only 25 watts with a reliable range of only about 15 miles. Many times it makes more sense to tune in a powerful commercial radio station, as shown in the Flight Guide Airports Directory, until approaching 15 miles out. This will give you an additional source of information to verify your destination. Then once in range reset the NDB to the compass locator frequency.

Better yet, back up the NDB with an RNAV waypoint. My Cessna 182 has a Northstar Loran unit. With this unit too, the NDB identifier can be set to obtain a heading to the compass locator. You can even set the secondary head of the unit on the airport identifier. Then on an ILS or NDB approach, cross check your track to the airfield.

While you may navigate to a terminal area using RNAV or GPS, generally pilots plan to land using an ILS/DME approach at airports that are properly equipped. After a long trip you may be tired and somewhat bored. Consequently it is very tempting to just dial in the ILS frequency, select Approach mode on the flight director and then just follow the V-bars to a smooth landing.

Yet in truth, this final phase of flight is where you must be especially paranoid. Dont trust anything. The accident statistics tell you why.

The most important reason is the potential for a false CDI lock-on. While every instrument-rated pilot knows about the false glide path potential on any ILS, this same potential exists on the localizer course.

Two possibilities are involved. First, you are 15NM out. As you are vectored to the final approach course, and well before you intercept the localizer, to get ahead of the game you select the approach mode on the flight director. Now, in theory, you are all set when the localizer becomes active and the glideslope comes alive. Yet in reality you are setting the stage for a false lock-on with either (or both) the CDI or glideslope.

The ILS localizer has a normal range limitation of 18 nm. From 10 to 18 miles its accuracy is guaranteed for only 10 degrees either side of the final approach radial.

The instructions for flight directors typically instruct you to capture the localizer in Navloc mode first, then switch to the approach mode. If you are still 20 to 30 degrees from intercepting the localizer, you are out of the envelope so to speak.

In approach mode, the flight director is more likely to show capture on a false localizer course. This is especially bothersome when the weather is at minimums and you are letting the autopilot fly a coupled ILS approach. With the compass locator properly tuned, you can verify your position and bearing relative to the localizer from the RMI or RNAV.

Once inside 10 nm, your cone for accurate off-course indications increases from 10 to 35 degrees. Now you can arm the approach mode early. Again, it is wise to cross-check the ADF bearing to the compass locator to be certain you are within the 35 degree envelope.

We all learned early on that the glideslope can give false indications. The time you are most likely to capture a false GS is when ATC has kept you high due to other traffic. The projection angle for a 3 degree glideslope intersects the outer marker at 1,400 feet and the middle marker at 200 feet. Thus when you cross the outer marker it is proper resource management to cross-check the altitude passing.

For example at Sacramento Executive airport when crossing Execc LOM your altimeter should be passing through 1,400 feet.

It is easy to get comfortable with the ILS and just fly the needles. But one cold IMC day, during an ATP training flight at Lincoln, Neb., an altitude crossing check saved the day. ATC had held us high to let an airline flight in underneath. Then once cleared for approach the pilot selected approach mode early and the FD locked onto a false glide path. We were alerted to the problem since we crossed the outer marker 400 feet low, but still showing on glide path. Out of curiosity we continued and broke out underneath the cloud deck at 600 feet and saw that the captured glide path would have put us at least a mile short of the runway.

When it comes to VOR, perhaps the main source of error is selecting the wrong frequency and then failing to check the station identifier. However in mountainous country it is possible to get CDI fluctuations that appear to signal approaching the station. In this case, use the To/From indicator to verify station passage.

Another VOR error that occurs occasionally in propeller aircraft is a plus or minus six degree fluctuation of the CDI. This is caused by the propeller RPM setting. Corrective action is simply a slight change in RPM, after which the CDI settles down.

Both VOR and ILS stations are identified by either (or both) a voice or Morse code identifier. When you hear a test code or no identifier at all then the station is undergoing maintenance and is unreliable.

This is especially critical for an ILS. Many times you will find the ILS identifier missing. Yet your CDI and glide slope indications appear normal. If the weather is bad, especially at night, do not rely on it. Several years ago an Air Force T-39 tried to fly an ILS at night in a low ceiling, using a facility that was NOTAMd as down for maintenance.

Upon arrival every thing seemed to work so they ignored the warning. Alas, at about 01:30 a.m. they touched down in the water, a couple of miles short of the runway at Eglin AFB in Florida. The two pilots were found sitting atop the floating aircraft at dawn the next morning.

DME errors in U.S airspace are very rare. One thing to remember is that DME gives you slant range and not actual horizontal distance. Thus if comparing DME mileage against GPS you will see a difference, particularly as you approach the station.

One symptom of malfunction you will see occasionally is a DME that continuously breaks lock then appears to lock onto a variety of distances; like it cant make up its mind. This probably indicates a malfunction of aircraft equipment, but in some areas it can be the ground station.

This is especially prevalent in developing nations. For example, after a two-year stint in the Philippines, flying a variety of airplanes, I became seriously paranoid concerning the trustworthiness of the ATC system there. Perhaps the best example to explain why involved the fatal crash of a Pan American freighter inbound to Manila International.

On the day of the Pan Am crash the weather was visibly dangerous, which was unusual for the Philippines. The most serious aspect was the extremely strong, gusty wind, with low hanging clouds covering the mountains to the north and east.

The high altitude arrival was from the north over the mountains. At the 20nm DME fix from Manila VOR/DME the aircraft should have been clear of the mountains. After passing the fix the flight was cleared to descend to a much lower altitude. But the Manila DME would break lock constantly, showing 20, 30, 60, then 10 nm. We had experienced the problem in the Embassy DC-3 so often that I distrusted the DME indication completely.

Unfortunately in the Pan Am flights case, the captain descended at the designated DME fix and hit in the foothills of the mountains, killing all aboard. Three weeks later a USAF technician from Clark Air Base was hired by the Philippine government to check the DME. After some maintenance, the DME was checked by a USAF inspection team and found to meet specifications.

The VOR/DME was then cleared of any fault in the crash; but Jeppesen was sued successfully for several million dollars for some minor missing notation on the approach chart.

The moral to the story is to use it, but be paranoid. Cross check continuously all radios and navigation systems. If you are flying into Mexico and Central America then a GPS is like life insurance. Dont leave home without it.

Aircraft Systems
Theres an old saying among aircraft accident investigators that DC dies and AC lies. What this refers to is that a sudden interruption in electrical power causes DC-powered instruments to drop to a zero indication. Conversely, AC-powered instruments will freeze at the instantaneous indication on the instrument. This obviously helps the investigator get a feel for the systems that were operating at impact.

To the pilot, this unique characteristic of freezing the indication can mean trouble. For example, in some fancier airplanes the ships oxygen bottle pressure gauge is AC powered. Thus during preflight, the pilot checks oxygen pressure and sees 1,800 psi, and thats just what it should be. Except without electrical power the indication merely shows what it was at shutdown the day before.

Perhaps the AC-powered fuel gauges are showing a fairly significant imbalance. Well, the rule of thumb for any malfunction in aviation is check circuit breakers. So you check the circuit breakers for both the left and right wing tanks. They look okay but you pull and reset each one, and behold; the gauges suddenly match. The high reading on one has dropped to match the other. For some reason you had lost power to one gauge (a faulty circuit breaker, perhaps) and the indication froze.

In another instance you are at cruise altitude and notice one fuel quantity gauge slowly decreasing and indicating an increasing imbalance. The engines are running fine, so it couldnt possibly be a fuel leak. Right?

You check the gauges again; even try the circuit breaker exercise. But the fuel imbalance seems to get worse. A check of your fuel pressure shows it fluctuating and somewhat low. In this case the instruments were trying to tell you that an engine has a serious fuel leak.

But you were skeptical because of the occasional freeze up described earlier. Now, when you reach an airfield, you want to shut down the engine with the mixture control before you change airspeed or begin descent. Otherwise you need to know the engine fire procedure.

Finally, theres the low or extremely high oil pressure indication. This usually occurs when you have a long over-water stretch or a 45 minute leg over some of the worst terrain youve ever seen. This can be a dilemma.

In my experience it is usually a false indication created by an oil pressure transmitter problem. In commuter and transport category airplanes, certification rules require at least two independent indicating systems. Thus with a low oil pressure you will see both the gauge reading low and an Oil Pressure Low annunciator light.

With both systems showing a pressure failure it has to be real and you must shut down the engine or risk severe damage and seizure. With a light twin having only the gauge, then it is prudent to simply shut down the engine to avoid damage and seizure. (Seizure usually precludes feathering; or in a single, a wind-milling prop.) In this case a bet that its a false indication can be real costly if you are wrong.

High oil pressure involves another gamble. In theory it implies that something is plugged and the all the bearing and gears are not being adequately lubricated. If you are about to fly across dangerous terrain, such as the north Atlantic or Gulf of Mexico, then its best to land and find the problem. Yet my experience has always been a false reading due to an oil pressure transmitter problem.

Cold weather is different. When it is cold, high oil pressure after engine start is a given, and you must wait for it to warm before running at high power settings or before departing. This is especially critical with turbocharged engines. Pre-heating the engine and oil is always a good idea in the winter. But with cold oil and high oil pressure your propellers may not be controllable during and after lift off.

Finally there is the erroneous RPM indication. If you are flying a Cessna 182 for example, the take off RPM must be 2,600 to give you rated power from the engine and the weight carrying capability you may be counting on. If one day you go to fly friends to a nice airport restaurant and on take off roll the prop RPM will only reach 2350, your first reaction may be Oh, its just the gauge. Chances are youre right, but if it is not the gauge then the airplane will not haul the certified load because it lacks the horsepower.

Too much RPM leaves you with the same dilemma. True, it may be the gauge; but if it is accurate you are flirting with engine failure. And too, you will have less than rated power.

Remember that you do need to trust your instruments, but it also pays to be paranoid. Just because everything looks normal, dont think that fluctuating fuel pressure is not going to start an engine fire.

Also With This Article
Click here to view “I Thought You Said This Thing Was Fast.”

-by John Lowery

John Lowery, a former Air Force and corporate pilot, is an aviation safety and training consultant.


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