Van's Air Force
Western Canada Wing
Let’s look at a real-world example.
You’ve flown from Langley up to Prince George with a friend to visit relatives.
On the way home, you decide to stop in at Barkerville (AS3) because you’ve
been told that nearby Wendle Provincial Park is really beautiful.
With your fuel load, passenger, and baggage, you’re tipping the scales
pretty close to gross in your RV-6, and it’s a warm summer day—24°C.
Are you safe to take off?
The table below summarizes the calculations we’ll use to answer that question.
|Field Elevation||4060 ASL|
|Headwind Component||10 kt|
|Runway Slope||2.15 %|
|Estimated Density Altitude||5950 ASL|
Let’s say that, in this configuration and weight, your 150 HP RV-6 would have a take-off roll of 550 feet at sea level. Using the density altitude rule, Barkerville’s elevation of 4,060 ASL is equivalent to 5,950 ASL. (Remember to reduce sea-level standard temperature—15°C—by 1.5°C per 1,000 feet. So Barkerville’s standard temperature is 9°C.) That alone will add 60 percent to your take-off roll, bringing it up to 880 feet.
Now you have to consider winds and runway slope. Let’s say the winds are from the east, so that runway 11 gives you a 10-knot headwind component. That would reduce your take-off roll by 10 percent, to about 790 feet. Unfortunately, runway 11 at Barkerville slopes up with a gradient of 2.15%. Should you use runway 29 instead, and take off downhill?
Well, the gradient rule-of-thumb says to treat 3 times the gradient like wind. So a 2.15-percent upslope is like a 6.45-knot tailwind, which is less than the 10 knots of actual headwind. In this case, it’s better to take off uphill. If the runway gradient was more than 3.33 percent, it would more than offset the 10 knots of wind and you should take off downhill and downwind.
But don’t make the mistake I once did! You can’t just take the 10 knots of headwind, subtract the 6.45 knots of “equivalent” tailwind, and assume you’ve got a 3.55-knot headwind. Remember, tailwinds have about five times the effect of headwinds. So calculate the wind effect first, then the slope effect.
Your take-off roll on runway 11 will be 1040 feet.
But we still haven’t answered the question: are you safe to take off? Another good rule of thumb is that the airplane should take off in the first half of the runway. If you’re half-way down and not yet flying, you should abort the take off. Runway 11 at Barkerville is 2,700 feet long. That’s comfortably more than twice your calculated take-off roll. So, at least so far as runway length is concerned, it looks like you’re safe. But don’t waste any of that 2,700 feet, start right at the button. And pick a “go/no-go” point halfway down the runway.
The RV-6 plans have climb rate plots for N66RV. They show a Vy (best rate of climb speed) of 120 mph, giving a climb rate of 1,500 fpm. They also show a Vx (best angle of climb speed) of 82 mph, giving a climb rate of 1,200 fpm. What’s the difference in climb gradient?
1,500 fpm @ 2 miles/minuteActually, either way you’re getting a pretty good climb gradient. At higher altitudes (such as on climb-out from Barkerville in the example above), you’d find a much greater difference in climb gradient between Vy and Vx.
= 750 feet per mile
1,200 fpm @ 82/60 miles/minute
~ 900 feet per mile
This might sound like an abstract idea, but it gives us a very simple tool for estimating small angles. For example, because a nautical mile is close to 6,000 feet (6,080), we can easily compute that the normal slope of an ILS final approach—3 degrees—is equal to 300 feet per nautical mile. Ten miles back on final the glideslope will be about 3,000 feet above aerodrome elevation. If you’ve ever been caught unaware by intercepting the localizer when you’re already above the glidepath, you’ll know how handy that calculation can be.
“I’m not an IFR pilot,” you might say. “Is the 1:60 rule any use to me?” You bet. It’s a really handy way to estimate wind drift when you’re using dead-reckoning navigation. Let’s say you’ve been following your planned heading of 090. Ten minutes into the leg (30 miles in your RV) you see, by reference to a ground feature, that you’re about a mile south of track. That’s 1 mile in 30, which is 2 miles in 60, or about 2 degrees off of track. Head 086 for the next ten minutes and you should be back on track. After that, 088 will hold the track, at least until the winds change. (This technique isn’t much use in the mountains, but it works great on the prairies.)
Another 1:60 ratio that’s really handy comes from the fact that there are 60 nautical miles in one degree of arc at the Earth’s surface. This means that you can convert the latitude markings on VNCs and WACs directly into miles. Each degree (of latitude) is 60 miles. (Note: this doesn’t apply to degrees of longitude, unless you’re at the equator.) Because latitude is shown in degrees and minutes, it’s easy to measure a distance on the chart with a pair of dividers (or your fingers!), compare it to the latitude markings, and get a distance measurement that’s accurate to within a mile!
If the lapse rate is more than 2°C per thousand feet, be extra-alert for signs of thunderstorms developing.
Lapse rate is the change in temperature
with altitude. The faster the temperature drops with altitude, the
less stable the air. Be particularly alert for lapse rates greater
than what was forecast. That means that the air is less stable than
the forecaster thought; the risk of thunderstorms is greater than forecast.
Whenever the lapse rate is more than 2°C per thousand feet, you’re
in relatively unstable air.
Thunderstorms are meanest on the side toward which they are moving, and the side from which they are fed.
In the northern hemisphere, thunderstorms are usually fed by southerly winds and travel eastward. So you can generally expect the worst conditions to be on the south and east sides of thunderstorms.
Thunderstorms are not all bad news, though.
CB clouds create strong, low pressures. Like all lows, air flows
into them following a counter-clockwise path. So, as with all lows,
if you pass a thunderstorm on the right you’ll get tailwinds. This
effect is strongest at altitudes below the base of the CB. Remember
to remain well clear of the thunderstorm area though—as in tens of miles
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