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When diversion to an alternate destination becomes necessary, the decision to change to a new heading should be made as early as possible. First, consider the distances to all available and suitable alternates, select the one most appropriate and then estimate the heading to the alternate chosen. Turn the aircraft to the new heading and then, more accurately, estimate drift correction, distance, speed, compass heading, and ETA while proceeding towards the alternate. Under the circumstances, it may be advantageous to “hold” over a known checkpoint while computations are made.
If a suitable choice is available, always divert to the spot that offers the easiest route to follow, such as a highway, river, power line, etc. This is particularly important if visibility is a factor. If low-level navigation is required due to low ceilings keep a careful lookout for obstructions. Diversion is often synonymous with emergency; this is not time to test your pure navigation skills if you don’t have to.
To compensate for the wind, which can often be an unknown factor in diverting to an unplanned alternate, establishing a heading by means of drift compensation may be the best expedient. The forward landmarks should be readily identifiable and, if visibility is a factor, they should also be reasonably close to the desired track. Again, rely on the instrument indications and any new heading established by drift compensation will be fairly close to the heading originally estimated for the diversion track.
In-Flight Mental Calculations
Mental arithmetic can be sometimes difficult when flying, and you may add to the following suggestions as your experience increases. When the distance and time is known, finding the ground speed is much easier if the “time” part is a fraction of 60 minutes. 25 NM covered in 12 minutes is 125 kts. We know that 12 minutes multiplied by 5 equals 60; thus, by multiplying the distance covered by 5, the ground speed can be easily determined. Another example is: 40 NM covered in 20 minutes is 120 kts (40 x 3). If a distance marker on a track line coincides with a checkpoint, the time at the distance marker twice the distance away will be twice the elapsed time.
If You Are Uncertain of Your Position
The danger of jumping to the wrong conclusion is very real when you cannot recognize expected landmarks. There are times when you will be uncertain of your position. Such moments require calm reasoning and a recognized procedure. The recognized procedure is as follows:
1. if you can, identify your last known position on the map;
2. hold a steady heading and check the heading indicator against the compass;
3. start reading “ground to map”;
4. climb, if possible; the higher you are the more ground you can see;
5. check all previous calculations by studying your chart and log;
6. draw a circle of uncertainty;
7. check for a possible wind shift;
8. try to get a bearing using radio aids; and
9. if all else fails find a suitable area and land until you sort out the problem.
Usually you can establish your position within a short time and continue the flight. You must never assume, however, that you are in a certain position. Check for a possible shift in the wind by drawing a circle of uncertainty.
To do this, draw a circle on the chart around the position you would be in if you were on track and on time. The radius of the circle should be 10 per cent of the estimated distance flown since the last confirmed position. Checks within the circle of uncertainty for any prominent physical features you have noted on the ground, and when you have positively identified one or more landmarks, fix your position. Remember, positive identification of a distinctive landmark is the only way to get back on track. If you decide that you are definitely lost, LAND. Do not exhaust your fuel in aimless wandering from one heading to another trying to pick up a landmark. You can usually find a landing spot, you burn a lot less fuel, and tend to think a lot more calmly and rationally when you are sitting on the ground. Always work to plan. Notify ATC by radio that you are lost, or try to contact any other aircraft or control agency. When you have established radio contact, transmit your general position, the amount of fuel remaining in the tanks, ask for whatever assistance you need, and indicate the action you propose to take.
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直升机飞行训练手册 HELICOPTER FLIGHT TRAINING MANUAL(42)
