Petite-Panel IFR

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You're in IMC and the electrics start to fade -- lots of clouds, no-coms, iffy navigation -- what else could go wrong? As one of AVweb's own recently discovered, you could lose your vacuum instruments as well. Features Editor Scott Puddy recounts a heart-stopping moment and discusses how you can keep the dirty side down if it happens to you, aided by some really cool graphics courtesy of AVweb's Linda Pendleton.


We were all trained to maintain control of an airplane in IFR following the failure of the vacuum system or one or both of the instruments that are traditionally powered by that system. The FAA Instrument Rating Practical Test Standards require it. We call it "partial panel." We exhort one another to fly partial panel more frequently to maintain currency in keeping the wings level relying solely upon the electrically-powered turn coordinator.

What if the turn coordinator packs it in as well? How about simultaneous failures of the electrical, vacuum and pitot-static systems? Could you maintain control of the airplane using only the magnetic compass? Could you fly "petite-panel" IFR?

It Could Happen To You

IFR WingLast October, I was flying our Bonanza from Seattle back to my home field in Concord, California. Engine start proceeded routinely and all instrument indications were normal during runup. Then, about 15 minutes into the flight after leveling at 7,000 FT MSL in IMC, I noticed a slight discharge on the ammeter. A quick check of all the switches and circuit breakers revealed no source for the problem. Cycling the alternator on and off did not help.

In the Bonanza only the bare-bones attitude indicator is vacuum powered. The turn coordinator, the HSI, the flight director, the auto-pilot, the Nav-Com's, the transponder (this occurred post-September 11 when operating transponders were MANDATORY), and the GPS (not to mention flaps and gear) all require 14 volts DC. An electrical failure is not a good thing. I told ATC that I needed an immediate descent to VFR and a vector for the ILS into Olympia. I switched everything off except for the GPS and the No. 1 Nav/Com.

ATC cleared me to 3,000 on an intercept heading and I lowered the gear to hasten the descent. The gear made it down on ship's power (but just barely). The GPS went black. I could receive but not transmit on the Com, but was still receiving the localizer. Olympia tower anticipated my arrival and gave me a green light as I approached for an uneventful no-flaps landing. My handheld transceiver restored communications for the taxi to transient parking.

The A&P traced the problem to a malcontent battery which he changed out the following day. We treated him to a quick test flight around the pattern. Less than 6 months and 100 hours following its installation, the vacuum pump failed on the downwind leg.

Oh my .... Had the battery lasted another half hour, had I been a little slower to detect the problem, or had the failure occurred at a less propitious location, I would not have been flying partial-panel IFR; I would have been flying petite-panel IFR.

Suddenly, the wisdom of training for the worse case failure scenario was obvious. If you can handle that, you can handle any lesser situation. In the case of instrument failures, the worse case scenario leaves you with no electrical power, a failed vacuum pump, an iced-over pitot-static system, and.... (we'll assume no engine fire because we want this to be realistic...).

Multiple systems failures are rare to be sure, but you are not INVULNERABLE; it could happen to you. If it does, it is no time for RESIGNATION; you can make a difference. If you can handle this, you can handle anything. It's doable, and here's how you do it...

The First Order Of Business: Survival

The key to survival in any spatial disorientation emergency is aircraft control. GA aircraft (excluding perhaps early-generation Cheyenne's) are longitudinally stable. However, GA aircraft are unstable about the lateral axis. As I discussed in an earlier article, left to their own devices most planes will put you into a spiral dive sooner or later. All that needs to happen is for the bank angle to momentarily exceed 30 degrees or so. From there, the over-banking tendency will pull the plane into a graveyard spiral. To avoid a spiral dive you need to be able to maintain wings-level and that is the essence of aircraft control.

The Whiskey Compass

Compass Characteristics

The above graphic shows a plane flying along a northerly heading somewhere in the northern hemisphere. The pilot then banks into a left (westerly) turn.  The compass magnets were initially pointed away from the pilot (toward magnetic north).  As the plane accelerates laterally, momentum tilts the compass card into the turn, the compass needles pivot downward to align with the magnetic dip and the compass incorrectly shows a course deviation to the east.

 

Here, the plane initially is headed south and the compass magnets point toward the pilot (toward magnetic north). The pilot then banks into a left (easterly) turn. The compass card tilts into the turn and the compass needles pivot downward, exaggerating the plane's progress in its turn to the east.

 

 Magnetic dip error disappears on headings of "east" or "west."  Here the plane is departing an easterly heading and the compass needles initially point to the pilot's 9:00 o'clock position (north).  The compass card tilts as the plane begins its turn but it does not pivot. The needles are already pointed downward as far as they can go.

 

The conventional instrument texts suggest that you can maintain wings-level using the standard-issue whiskey compass merely by keeping the compass from turning. That is so but it isn't as simple as it sounds. As a starter, it is helpful to have a good understanding of compass construction and compass characteristics (a.k.a. "errors").

An aircraft magnetic compass is an unpretentious device consisting of a sealed outer case containing a pivot assembly that supports a floating compass card. Attached to the compass card are two or more bar magnets that orient the compass card to the north. The card rotates freely and can tilt up to 18 degrees. The case is filled with an acid-free kerosene that dampens oscillations and lubricates the pivot assembly. The pivot assembly is spring-mounted to dampen aircraft vibrations so that the compass heading is more legible.

Reading A Magnetic Compass

People accuse the magnetic compass of rotating backwards because they don't understand it. Fundamentally, the compass card doesn't turn at all. It just sits there pointing northward while the airplane rotates around it. If you can visualize that, you can readily determine whether you're turning left or right. If the compass card appears to be rotating clockwise (but you know that it's actually stationary), you can visualize that the airplane is turning left.

If that doesn't work, compare where you were to where you're going. If you were headed east a moment ago and are approaching a heading to the south, you're turning right. Imagine yourself standing in an Iowa cornfield if that helps. "Let's see, I was headed toward New York but I'm turning in the direction of Mexico...that would be a right turn."

If that doesn't work, you're turning right if the numbers are getting larger; you're turning left if the numbers are getting smaller.

If that doesn't work either, remember that the compass rotates backwards.

Magnetic Dip/Tilt/Pivot Error

Compass errors include: variation, deviation, oscillation and magnetic dip. The last of these, which might be re-titled "magnetic dip/tilt/pivot" error, is the one that significantly affects controllability.

"Magnetic dip" is a reference to the vertical orientation of the earth's magnetic field. As with a bar magnet, the lines of force pass through the center of the earth, exit at both magnetic poles, and bend around to re-enter at the opposite pole. At the equator the field is essentially parallel to the earth's surface. At greater distances from the equator, the magnetic field dips increasingly toward the earth's surface. The bar magnets in an aircraft compass will attempt to align themselves horizontally and vertically with the earth's magnetic field. Anywhere in North America, if unconstrained, they would point northward and downward.

However, the aircraft compass bar magnets are constrained because they are attached to the floating compass card. In all phases of flight, the compass fluid and compass card will sit perpendicular to the total G-force acting on the plane. In unaccelerated flight, the sole G-force (gravity) pulls straight down. The compass card will be level and the magnets can rotate along only the horizontal plane. Thus constrained, they will align themselves as closely as possible with the magnetic field by pointing toward magnetic north.

"Tilt" occurs in accelerated flight because the force opposing acceleration (a.k.a. "inertia" or "momentum") pulls on the fluid and compass card like the moon pulls the tides. In a turn, momentum pulls the fluid and compass card to the outside of the turn. Objects in motion tend to stay in motion and the internal compass components will attempt to continue along a straight path as the horizontal component of lift accelerates the plane through the turn. Pulled outside the turn, they tilt into the turn.

Contrary to a common misconception, the airplane's bank angle causes no compass error except insofar as it correlates with rate of turn. You can prove that to yourself in a car with a clear-glass, liquid-filled container. As the car turns left it will not tilt into the turn, but the liquid will. You can also experiment in an airplane using the same liquid-filled container. In a turn the liquid will tilt into the turn. However, in a forward slip (bank but no turn) the liquid will remain level. G-force, not bank angle, is the culprit here.

Changes in speed have similar ramifications. As an airplane accelerates forward (speeds up), inertia pulls back on the fluid and compass card. They tilt forward. As an airplane decelerates (slows down), momentum pulls the fluid and compass card forward. They tilt back.

"Pivot" occurs whenever "tilt," by enabling motion along the vertical plane, allows the compass magnets to align themselves more closely with the earth's magnetic field by pivoting left or right.

The error is most pronounced upon turning from a heading of north or south. On a northerly heading the compass magnets are pointing to the pilot's 12:00 o'clock position (north), situated horizontally in line with the longitudinal axis of the airplane. As the plane begins a left turn, the fluid and compass card will tilt to the left. The compass magnets will still be horizontal and oriented toward 12:00 o'clock (north). Now, however, they are free to move vertically. Capitalizing on that opportunity, the magnets will pivot downward, counter-clockwise into the turn, and the compass will falsely accuse the plane of initiating a turn to the right.

On a Southerly heading, the compass magnets will rest horizontally, pointing to the pilot's 6:00 o'clock position (north). As the plane starts a left turn, the fluid and compass card again will tilt left. Given the opportunity to point downward, the magnets will rotate the compass card clockwise, exaggerating the plane's progress in its left-hand turn.

There is no turning error upon turning from a heading of east or west. For example, on an easterly heading the compass magnets will be positioned horizontally at the pilot's 9:00 o'clock position. As the plane rolls into a left turn, the compass, the fluid and compass card will tilt left and the compass magnets (which are pointed toward the direction of tilt) will point downward to the maximum extent permitted by the tilt angle. Since the compass magnets are already pointed north and there is no opportunity for the compass magnets to point further downward, the compass card will not pivot except to the extent that airplane actually changes heading.

Two Sides and Two Safe Havens

It should be apparent from the above that a magnetic compass has a dark side, an embellished side, and two safe havens. The dark side is to the north. If you depart wings-level from a northerly heading the compass will recommend the wrong control input, which could lead to a loss of control. The embellished side is to the South. If you depart wings-level from a Southerly heading, the compass will overstate your turn. That can lead to over-corrections, pilot-induced oscillations and a loss of control. The safe havens are east and west. There the compass will respond correctly and proportionately to the airplane's rate of turn.

Flying Petite Panel IFR

IFR WindshieldSo...the entire panel just went black. What's a pilot to do?

The first three steps in responding to any in-flight emergency are: 1) Fly the airplane, 2) Fly the airplane, and 3) Fly the airplane. Tell everyone (including ATC, Muffy and Junior) that you're busy handling an emergency right now. You'll fill them in after a few minutes.

The panel is a mess and you're disoriented. Anything that you do with the yoke will probably be the wrong thing. It's time to give the airplane its head. Return the ailerons to neutral and let go. If the wind noise is beginning to sound like you've departed Kansas for Oz, you're probably also going too fast and it's time to chop the throttle and drop the gear.

Fold your hands in your lap and study the compass. When you actually experience a multiple systems failure, you'll probably be turning left or right by the time all the shouting dies down. That is not altogether a bad thing. The compass can mislead you concerning your direction of turn only as the airplane commences a turn. Once the airplane is established in a turn, the compass can give erroneous static information concerning your heading at any given moment. However, the compass will give accurate trend information. It will reliably report your direction of turn through 360 degrees. Using any of the methods discussed above, or any combination of them, determine your direction of turn.

Next, assess your rate of turn. The compass changes 10 degrees every 6 seconds in a half-standard rate turn. If it's spinning faster than that, apply opposite rudder to shallow the turn.

Your goal is to establish a gradual turn in a known direction until you encounter "W" or "E" on the compass card. That's your safe haven. Once you get there, stop the turn with opposite rudder and wait for the wind noise to die down. When it does, reconfigure the plane for a slow cruise. The plane was stable before the panel went black and should be stable afterwards so long as you maintain wing-level and all the control surfaces are still attached.

You learned to "step on the ball" during private pilot training. Here the mantra is to "step on the line." Whenever the lubber line drifts right or left of "W" or "E," apply rudder on that side to re-center your safe-haven identifier. (Don't worry about committing that to memory. You'll quickly figure it out with a little experimentation.)

Once you've satisfied yourself that you can hold a heading of "W" or "E," you have regained control of the airplane. It's time to reassure Muffy and Junior, contact ATC, and obtain a vector to VFR (if you don't already know where it is). Your preference is to head due east or due west, but you can accept a heading to the south.

Avoid northerly headings if you can. If north is the only way out of Dodge, remember that you're on the dark side of the compass. If you initiate a turn to the left the compass will initially show a turn to the right. However, if you allow the turn to continue, the improper indication will correct itself and the compass will begin to show a turn to the left. So....if you're headed to the north you need to demonstrate patience. You can't react to the compass's initial indications. You have to wait it out. It's doable, but it's dicey.

If you ever begin to doubt your ability to control the airplane on a northerly heading, start over. Initiate a slow turn in either direction until you encounter a safe haven ("E" or "W"). Regain control. Regain confidence. Reestablish communications with ATC and request a heading away from the dark side.

Like any other emergency procedure, it is best to rehearse this process before you have to do it for real. So ... the next time you're out flying with a hood and a half-dozen spare instrument covers in search of something to practice ....

Fly safe.

-- Scott


About the author...

R. Scott Puddy is an ATP, CFI-I, MEI who teaches out of Livermore Airport (LVK) in Livermore, Calif. Scott is type-rated in the Beech/Raytheon King Air 300 series but regularly flies a V35 Bonanza or a B55 Baron and practices law in San Francisco.