Instrument Flight – by Richard Morey, CFII
In my last column, we discussed the issues associated with vacuum pump maintenance. Even with good maintenance practices, you may still experience a vacuum pump failure. Being prepared for this by practicing partial panel instrument procedures could literally save your life and those of your passengers.
Recognition & Its Aftermath…
Unusual Attitude Recovery Partial Panel
When you experience a vacuum pump failure, your gyro instruments begin to spin down. Both artificial horizon and directional gyro will begin to give erroneous information. Unless you are lucky or have a low vacuum warning light on your ship, you will not catch the vacuum failure instantly. Normally the first indication of a vacuum failure is when you discover that the artificial horizon does not match up with the turn coordinator, vertical speed indicator and altimeter. This realization generally comes after you have followed the erroneous artificial horizon into an unusual attitude.
Let us assume the worst case. You realize that your artificial horizon does not match up with reality. Cross-checking your gauges confirm this. You are in an unusual attitude partial panel! The most important thing is to remain calm, assess the situation and be smooth on the controls during recovery. This is easy for me to say, but with training and practice, the process becomes automatic.
Unusual attitudes come in two types: nose high and nose low. Recovery technique depends on which of these has occurred. You first need to interpret the gauges and determine which type of unusual attitude. Many people, when they find themselves in an emergency situation, feel the need to tell Air Traffic Control (ATC) immediately. Remember, in an emergency you may deviate from the regulations to the extent needed. Always fly first; you will have plenty of time to talk later.
If the vertical speed and altimeter show a climb, and your airspeed is decreasing, you are in a nose-high attitude. To recover, your eyes go to the vertical speed indicator. Gently press the control yoke forward until you see the vertical speed needle reverse itself. This is enough pitch correction. If you push forward until the needle reads zero feet per minute, you will be in a dive.
Remember, the vertical speed lags for rate, but is instant for trend. Add full power as you press forward on the yoke, then your eyes should go to the turn coordinator. With rudder and aileron, level the aircraft’s wings. Although the turn coordinator does not give attitude information directly, if your ball is centered and the “wings” of the turn coordinator are level, then your wings are level.
Verify level flight with the vertical speed indicator, verify wings level with the turn coordinator and ball, and then throttle back to your normal cruise power setting. If you were trimmed for level flight before the failure, returning to your previous power setting and airspeed will result in a level attitude, assuming you keep the wings level.
If the airspeed indicator shows an increase in speed, and the vertical speed indicator and altimeter both show a descent, you are in a nose-low attitude. To recover, you first reduce the throttle to idle, then roll your wings level by reference to the turn coordinator. Then gently apply back pressure until the needle of the vertical speed indicator reverses itself. Verify straight and level as in the nose-high recovery, and increase throttle to the cruise setting. Again, if you were trimmed level prior to the failure, returning power and airspeed to prior levels will result in level flight.
Try both of these recoveries in visual conditions without the hood first. Pay close attention to the vertical speed and turn coordinator. You will see what I mean when I say apply yoke pressure just until the vertical speed needle reverses itself.
The next step is practicing full panel recoveries with a safety pilot. This should be accomplished in simulated instrument conditions (under the hood). Once you are comfortable with the sequence of recovery, try covering the artificial horizon and do the procedures partial panel.
I start out with my students flying themselves into the unusual attitude. Once they are proficient, I have them close their eyes, and take their hands and feet off the controls. I then put the aircraft into an unusual attitude and allow the student to recover.
Practice unusual attitude recovery partial panel until you are not only comfortable, but confident in your ability to recover. If you find yourself getting queasy, stop the maneuver, pull your hood and spend some time looking at the horizon with the air vents open. You cannot learn, when you are airsick!
After you are comfortable with “simulated” unusual attitudes, you should, if at all possible, schedule some time in a simulator. Instead of having your instructor or safety pilot declaring that you have had a vacuum system failure, and covering your artificial horizon and directional gyro, your instructor will simply fail the vacuum system by inputting a command into the computer running the simulator. Now you will see firsthand what following an erroneous artificial horizon is like. You simply cannot simulate this in an airplane.
Declaring An Emergency
Don’t even think about making a radio call until you have the aircraft stabilized. We, as in “we humans,” cannot talk and think at the same time, let alone fly a partial panel unusual attitude recovery. If you have an altitude deviation, so be it. If ATC calls, ignore them until you have the aircraft stabilized. Never drop the airplane to pick up the microphone! Do not let anything distract you from the task at hand, which is flying the airplane. You will have plenty of time to fill out paperwork on the ground if requested.
Keep a NASA form for voluntary reporting in your flight kit. Fill it out and send it in even if ATC does not request a report. Doing so will establish your cooperative attitude to the FAA. It is not a get-out-of-jail free card, but it will help if any questions arise from the situation.
Once you have the aircraft stabilized, then you can communicate your situation to ATC. If you are in Instrument Meteorological Conditions (IMC), you need to declare an emergency. When you declare an emergency, be very specific. Tell ATC that you are “no gyro.” If you tell them you lost your vacuum pump, they may or may not know what you mean; but controllers are trained to understand “no gyro.”
Know The Weather
Recognizing the vacuum failure and stabilizing the aircraft are just the beginning. You now need to either fly to visual flight conditions or fly an instrument approach procedure. Knowing the weather conditions will help you decide which.
For instance, if the closest airport has visibility at ½-mile and a 200 ft ceiling, and on the other side of the front, 40 miles away, the weather is clear, the decision is easy. If within the range of your aircraft, the weather is uniformly bad, your decision is made for you. Usually though things are not so clear cut.
Is ASR An Option?
Airport Surveillance Radar (ASR), if available, is perhaps the best choice for a partial panel approach. Not all airports have this type of approach available. If you use government charts, look under “Radar Minimums,” pages N1, to find a list of airports and minimums for the ASR approaches available in that area. When you do your preflight planning, you may consider listing an airport with an ASR approach as your alternate, or at least know where the nearest ASR approach is located relative to your route.
ATC or approach control are resources you should use. In an emergency, they will make it a priority to get the information you request. If unsure, ask!
An ASR approach is essentially a series of vectors to align your aircraft with a runway. In an emergency you will be assigned a “dedicated” controller and radio frequency. No other aircraft will be on the frequency assigned, and the controller’s only responsibility will be to handle you. Remember to request a no gyro approach. Your controller will tell you what minimum altitude to expect, and later when to start your descent. The controller will also start off assuming standard-rate turns, and will issue instructions on that basis. Later the controller will tell you to use half-standard turns and eventually request that you not respond via the radio. Expect instructions such as “start left turn, stop left turn.” The controller will be timing the turns. A standard rate turn is 3 degrees per second, so if the controller wants a 30-degree turn to the right, you will receive instructions to “start right turn” then 10 seconds later be told to “stop turn.”
If you are lucky enough to live near an airport that offers ASR approaches, I would recommend that you incorporate them as part of your instrument currency. Just as pilots need to log approaches to stay current, so too do controllers. Requesting a practice ASR (no gyro) approach is often met with enthusiasm, especially if traffic is light and it is close to the end of the month.
Flying your first ASR without a hood is not a bad idea. Later fly with a safety pilot and view limiting device full panel, and finally with the artificial horizon and directional gyro covered to simulate vacuum failure.
The Importance of Trim & Knowing Your Numbers
Flying an instrument approach partial panel is not particularly hard, but it does require different techniques and instrument interpretation. This becomes much easier if you have the aircraft trimmed and know your performance numbers.
For example, take the Cessna 172SP. On our’s 2050-2100 RPMs will result in about 90 knots when trimmed for level flight. 1800 RPMs (and a slight bit of left rudder) will result in a 500 foot per minute descent, again at about 90 knots. 1500 RPMs gives us about a 1000 foot per minute descent, again at a bit above 90 knots. To level off after a descent, don’t pull back on the yoke, but rather start increasing your throttle to 2100 RPM 50-100 feet above your target altitude. Remember to add a bit of right rudder. The aircraft will level off and remain on its heading if you do your part. All this is accomplished without touching the elevator trim. This assumes the aircraft is trimmed correctly to begin with, and you are not inadvertently adding incorrect elevator pressures.
Flying With Your Feet
By letting go of the yoke, and simply flying with rudder, you guarantee that you will not accidentally add elevator pressure. You can fly your aircraft with rudder pressure and power changes assuming the aircraft is trimmed correctly. Your focus should be on the turn coordinator. Keeping the turn coordinator’s wings level will result in holding your heading. A bit of left or right rudder pressure will result in a turn. You can regulate the turn’s rate just by rudder pressure. Again, this is best practiced initially without a view-limiting device and in visual flight conditions. Try it; it really works!
The Turn Coordinator Is Your Friend!
In partial panel instrument flight, 80-90% of your scan should be spent on the turn coordinator. If you wish to hold a heading, be sure the “wings” of your artificial horizon are perfectly level. If you maintain this, the aircraft will hold its heading. Of course you do need to check altimeter, vertical speed indicator and compass as well as your course deviation indicator, but your eyes should return to the turn coordinator afterwards.
The turn coordinator also is essential for establishing and ending a turn. With the turn coordinator’s wing centered on the turn reference, you will be turning at 3 degrees per second, if the turn coordinator is in calibration. Turn coordinators do go out of calibration though, so be sure to check your unit by timing a 360-degree turn, both left and right. You should be two minutes +/ – 10 seconds. If you put the wing half way between the no turn reference and the standard rate, you have half standard or about 1.5 degrees per second. If you find that you have exceeded the standard rate reference, you have no idea how many degrees you are turning per second.
Compass Turns
With a vacuum system failure, your directional gyro and artificial horizon, if vacuum operated, become inoperative. It is important to cover these gauges as they offer confusing and erroneous information that is hard to ignore. I keep sticky notes in my flight kit, but in a pinch, a dollar bill folded in half and slipped behind the trim, can work as well. Larger denominations work, but can be distracting to safety pilots and especially flight instructors!
Without a functional directional gyro, determining your heading becomes a bit more complex. Having a GPS on board greatly simplifies this task and makes partial panel approaches much easier as well. Without a GPS, we need to rely on the compass. Understanding the limitations of the magnetic compass is necessary for good results. Because of the nature of magnetic lines of force, there are times when the magnetic compass will not give you correct heading information.
Try this the next time you are out flying.
Start a standard rate turn to the left from an east heading. Pay attention to both the compass and directional gyro. We all know that the compass is “backwards,” as we have to turn away from the direction (as depicted on the compass) you want to turn towards.
This is the first challenge when using the compass. You will notice that for the first 30 degrees or so, the compass and directional gyro pretty well match up. With some compasses though, a standard rate turn will “hang up” the compass, keeping it from turning freely. If this is the case you will have to use half standard rate for your compass turns or spend the money on a new compass. Half standard rate in compass turns is quite acceptable and some feel it to be the better technique.
Continuing the turn, you will notice that the compass will start pulling ahead of the directional gyro, leading it by (in the Midwest) 30-40 degrees by the time you are on a north heading. This is the UN portion of UNOS, as in “Undershoot North.” As you continue past north, you will see that the compass is turning slower than the aircraft.
On a west heading both compass and directional gyro should read the same. Continuing towards south, you will see the compass lagging behind the turn.
On a south heading the compass will indicate 30-40 degrees under. This is the OS portion of UNOS as in “Overshoot South.” By the time you return to east, the compass and directional gyro should match. If you were to roll out of the turn on north or south, and if your wings were precisely level, then the compass would read correctly. If you are even slightly banked, the compass will not be accurate.
I teach my students to only check the directional gyro against the compass when on an east or west heading to avoid, or at least minimize, compass error. Turbulence makes reading the magnetic compass more challenging. There are times when you simply have to take the average of the compass swings as your heading.
Let’s not forget the acceleration and deceleration errors. If you are on a north or south heading, this type of error is minimized. If you are on an east or west heading, and you are accelerating, the compass will show a turn towards north; if decelerating, a turn towards south. The mnemonic here is ANDS, as in accelerate north, decelerate south.
What all this means to you in a partial panel situation is that unless or until everything mentioned above makes complete sense to you, to the point of not having to think about it, you should not rely completely on compass turns for heading when flying partial panel. Unless you grew up flying aircraft without directional gyros, getting to that level of compass knowledge takes much practice. Fortunately, there is another option.
Timed Turns
By using the turn coordinator and a timer, you can make very precise heading changes. Without the directional gyro, you need to break your actions down into its most basic parts.
First, determine your heading. Remembering the compass errors, make sure your wings are level, and that you are not under acceleration or deceleration. Then read the compass.
Next, determine the direction of the turn. Again the compass is “backwards.” You have to turn away from the compass. “Right raises and left lowers.”
For example, let’s say you are on a heading of 100 degrees and you wish to fly a heading of 130 degrees. Since you wish to “raise” the heading from 100 to 130, this is a right hand turn. Now you need to figure how many seconds this will take. 30 degrees at 3 degrees per second is, of course, 10 seconds. But doing math in your head while flying partial panel, is a distraction you do not need.
For larger turns, I use my VOR/Localizer Course Deviation Indicator (CDI) as an aid. If your aircraft has an Automatic Direction Finder (ADF), you can use its associated indicator as well.
The CDI’s compass rose is marked every 30 degrees, or every 10 seconds at standard rate. Visualize your heading and the heading you are to turn to on the compass rose of the CDI. Now count the 30-degree hash marks between your actual heading and that desired, and note if it is a right or left hand turn. Double check your compass to make sure you did not drift off your heading and then roll into the turn at standard rate and start timing.
Use a timer or the sweep second hand of the aircraft’s clock if the turn is more than 10 seconds. For turns under 10 seconds, I simply count. When sufficient time has passed, roll out of the turn referencing your turn coordinator, let the aircraft stabilize and check the compass.
You may need to adjust your heading slightly, or perhaps a bit more, depending on how well you held standard rate in your turn. If you made the common mistake of turning in the wrong direction, don’t panic, just recalculate your turn and correct the error. If you find yourself over standard rate in your turn, your best course of action is to roll out of the turn, wait for the compass to settle down and figure out your heading, then recalculate. For very small corrections simply roll into a standard rate coordinated turn in the correct direction and then roll out of it. This will give you anywhere from 3-15 degrees of turn depending on your personal sense of timing.
Tracking
You will need to track both for enroute and approach segments. If the CDI is at full deflection, decide on an intercept heading and turn to it. Be patient and remember that an intercept angle of 30 degrees or less should be sufficient. For enroute segments, request “direct to the VOR” to simplify your task.
Once you have the CDI’s needle centered, check your compass to make sure your heading matches your setting. If you needed a correction angle to track your course prior to the vacuum failure, you will still need it. The secret to tracking partial panel is to hold your heading and watch the trend of the Course Deviation Indicator’s needle. Holding a heading is simple if you spend most of your scan on the turn coordinator and crosscheck the compass.
If you see the needle trending towards the right, simply roll in and roll out of a standard rate turn to the right. Now hold the heading and see what the needle does. If it stops moving, roll in and out to the right once more. Hold this heading and the needle should start moving back towards the center. When it centers, roll in and out to the left. This should set up a correction angle. Again, if you hold your heading and watch the trend of the needle, rolling in and out in the direction of the needle’s movement should enable to track a radial easily. Within 3 miles or so of the VOR or inside the outer marker on a localizer, rolling in and out to half standard rate gives you a smaller correction.
GPS Makes It So Easy!
(If You Practice)
If you are familiar with its function, a panel mounted or yoke mounted GPS greatly simplifies flying partial panel approaches. Having a near-instant readout of your heading eliminates dealing with the compass. If your GPS has a moving map, your positional awareness is greatly enhanced as well. If you do not practice using the GPS in partial panel simulation, you could find yourself behind the airplane and the GPS. Not a situation you want to be in!
Standby Vacuum Pump Systems & Other Options
If you would rather not have to deal with a vacuum failure, a standby vacuum system may be in order. Both the electric and the manifold type work well if you understand how they operate. An electric artificial horizon or directional gyro or both make vacuum pump failure less of a problem, but remember electrical systems can fail as well.
It ultimately comes down to balancing risk and expense. The most cost-effective way to minimize your risk is and always has been to exercise good judgment and plan your flight with all that could go wrong in mind. If you keep your skills sharp, and plan for emergencies, you have a much greater chance of a good outcome when something out of the ordinary arises. Practice may not make perfect, as perfection is beyond us mere mortals, but practice does make the master!
