The Mechanical Flight Computer -- Part 2

Have you ever had to choose a cruising altitude based on variable wind directions and speeds? Certainly you'd want that altitude to give you the highest groundspeed for your course. How do you make that choice? You can make an eyeball estimate but that's not always accurate enough. Instead of casting your fate to the wind (so to speak), why not try a more scientific method, like using the wind side of the E6-B computer shown in Figure 1. Here's how to do it.

Figure 2 shows the winds aloft forecast for today's flight.

Let's assume your true course is 260 degrees and true airspeed is 156 knots. On the clear plastic portion of the circular scale plot the wind forecast for each altitude shown in Figure 2. Do this by placing the grommet (the center) on the airspeed arc labeled 100 (Figure 2A). Place the wind direction under the true index by rotating the circular scale. Then mark the wind velocity up from the 100 arc.

Figure 3 plots the wind for 3,000 feet.

Figure 4 plots the wind for 6,000 feet.

Figure 5 plots the wind for 9,000 feet

Figure 6 plots the wind for 12,000 feet.

Now rotate the circular scale to place the true course (260 degrees) under the true index as shown in Figure 7.

To determine the wind's effect on your groundspeed, position the 156 knot true airspeed arc under each wind dot. The groundspeed for that wind is shown under the grommet. Let's test each wind dot. Figure 8 indicates that the wind at 3,000 feet produces a groundspeed of approximately 141 knots.

Figure 9 indicates that the wind at 6,000 feet produces a groundspeed of approximately 153 knots.

Figure10 indicates that the wind at 9,000 feet produces a groundspeed of approximately 162 knots.

Figure 11 indicates that the wind at 12,000 feet produces a groundspeed of approximately 164 knots.

It seems that the best groundspeed (164 knots) occurs at an altitude of 12,000 feet. Since you can get a groundspeed of 162 knots at 9,000 feet you may consider this altitude as an option. You need to weigh the benefits of climbing an extra 3,000 feet just to go 2 knots faster. Take a look at your performance charts and examine the difference in fuel consumption between these altitudes. Consider that flying at higher altitudes suggests the use of supplemental oxygen (i.e., recommended for flights above 10,000 feet during the day or 5,000 feet at night). Additionally, my experience tells me that when wind changes speed more than 4 knots per 1,000 feet, you're likely to encounter wind shear. Between 9,000 feet and 12,000 feet, the wind changes 30 knots, which works out to a 10 knot change per 1000 feet. It's safe to say that it will be bumpy between these altitudes. This is one more reason you may elect to stay at 9,000 feet instead of 12,000 feet.

This is a handy little technique for use during preflight planning or even while in flight. But, then again, I tend to like the mechanical flight computer because it lets you visualize the effect of wind on your airplane. I know of no electronic flight computer that provides a similar graphic image. The moment one does, I'll gladly retire my E6-B and make a big fuss about this new electronic marvel.