Compass
The compass is the least likely to fail and any change in its numbers would indicate the opposite direction of turn. If ever the turn coordinator disagrees with the attitude indicator use the compass to break the tie. Heading indicator as a vacuum partner with the attitude indicator is a biased juror.

This material is included in the VFR material but is repeated because of the partial panel requirements of the IFR PTS. Mounted on the face of an aircraft compass is a chart. This chart is a record of compass error called deviation. As a pilot of a particular aircraft you should copy this chart for use in navigational planning.

Many airports have an area set aside for a compass rose. This rose is aligned using the variation between the True North and Magnetic North for this specific area. It shows the magnetic directions such as are used to align the airport runways. It is a place where aircraft are positioned and 'swung' through the various magnetic directions. This 'swinging of the compass' is done with all electric circuits functioning so that the operation of the compass will reflect this fact when flying.

As the aircraft is positioned on the eight magnetic courses a small pair of adjustable magnets are moved so as to get the most accurate compass reading possible. As these adjustments are made, a record is made of the compass direction of the aircraft on the rose as compared with the best-adjusted reading of the aircraft compass. This record is transcribed on to the deviation chart affixed to the compass.

In most cases this deviation is only one or two degrees. This exceeds the straight line flying ability of a pilot. However, with the advent of the Global Position System, it is becoming important to note deviation as a factor in navigation beyond the Practical Test Standards or the FAA written.

The numbers on the compass are opposite in order and direction from the HI. The reversal of the two numbering systems requires us to be consciously aware of the difference. Setting the compass to the HI requires that we note the compass lubber line is between two numbers either centered or near one than the other. These two numbers are then located on the HI and the HI set to correspond. A usual student error is to make a mistake setting the compass.

Flying with the compass is quite different from using the directional gyro. The compass has several inherent errors relating to turn, speed and geographic latitude. The compass should only be "read" in level un-accelerated flight. It is best (easiest) to make compass turns using the turn coordinator and time. At 3 degrees a second a turn of two minutes is 360 degrees, one minute 180 degrees, 30 seconds 90 degrees and 10 seconds 30 degrees. A normal count of 1, 2, 3, (4) will be close to 10 degrees. On the ground a compass should only be checked while taxiing straight or when stopped at a known heading.

The swinging and dipping of the compass during a turn or acceleration due to changes in speed or direction is a physical phenomenon caused when the north seeking end of the compass dives toward the north magnetic pole. The higher the latitude the greater the dipping tendency. Turns from a northerly heading lag behind the turn; turns from a southerly heading lead the turn. When the card is banked the compass dips to the low side of the turn. ANDS is the mnemonic for acceleration errors. In a shallow 360 turn the compass is most accurate at 90 and 270 degrees. In a smooth turn from a south heading the rate shown on the compass exceeds that of the actual turn at a diminishing rate until at 90 or 270 degrees. See AC 61-27C pg. 44.

In the planning of a flight the use of the mnemonic "True virgins make dull company" gives the order true course, variation, magnetic course, deviation, compass course, wind, wind correction angle, to provide the compass heading required for the flight. In level un-accelerated flight the compass is unaffected by turning or acceleration errors. The compass is the self contained and independent means of determining flight direction.

Visibility
Visibility Defined::
FAR 91.175(c)(2) defines visibility prescribed for an approach.
FAR Part 1 defines both flight and ground visibility, and there Is a difference in the two meanings.

Determining Visibility
--Decide what you will look for before beginning the approach.
--Study the approach lighting and runway length.
--MALS lighting system is 1400'; MALSR is 2400'
--Learn to count runway lengths to the airport as a means of determining distance..

Part 91 Visibility Factors
Part 91 takeoffs have no minimums but...
Part 91 landings have three prerequisites
1. Normal descent
2. Minimum visibility
3. Runway in sight

You cannot descend below DH or MDA unless you have minimum flight visibility required by the approach plate. This determination of minimum is done by the pilot. The IFR training program does not teach you how to determine visibility. Reliance on a 45 minute old ATIS is likely neither valid nor reliable. The most difficult phase of any approach in actual conditions is where the transition for instruments to visual actually occurs.

You will be at the decision height about an eighth of a mile before you should see a runway at ILS minimums of a half-mile. Seeing the runway before reaching decision height means you have the required visibility. Seeing the approach lights but not the runway allows you to continue but the missed is your best option. Knowing that each group of lights are 200' apart gives you a handy distance reference to the runway. If you see all of the MALSR you have required visibility. Seeing all of a MALS system is less than a half-mile of visibility, go-around.

Flight Visibility:
Average forward horizontal distance, from the cockpit of an aircraft in flight, at which prominent unlighted objects may be see and identified by day and prominent lighted objects may be seen and identified at night. Flight visibility is determined by pilot. A pilot may not land an aircraft unless the flight visibility is as prescribed in the approach procedure.

Ground Visibility:
Prevailing horizontal visibility near the earth's surface as reported by an accredited observer. Reported ground visibility has no reflection on actual flight visibility. Landing and takeoff visibility are ground-based measurements.

Visibility above the minimums is a two-way street. It makes the takeoff safer but does not, necessarily assure a safe landing. Regardless of conditions, planning and flexibility are the keys.

Low Visibility IFR
Zero/zero takeoffs are in themselves not particularly hazardous.

Its the ‘what-ifs' related to engine failure on takeoff or having an immediate need for an alternate landing field that makes such takeoffs ill advised. A straight-ahead landing requires that you know what is there even though you can't see. You must review and know the departure area to gain even some element of survival.

Low visibility takeoffs require that you maintain runway headings and low visibility landings require a minimum visibility. A Part 91 pilot can depart in zero/zero but must have visibility minimums and the runway environment in sight before descending below MDA or DH. What you hear on the ATIS is not controlling. It's what you see.

Clouds or rain are most often as stated. Fog can be variable and change rapidly. If fog is a factor be prepared to go to an alternate. Use a second pilot to call outside the cockpit while the first pilot stays on the instruments. In single pilot operations the greatest hazard in such conditions is to let your sink rate increase while you are looking for an opportunity to dive for the runway. Don't!

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