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Fly Safer With These Handy Tips

by Tedd McHenry
This article was inspired by a talk given by John Laing, an ATP- and CFII-rated pilot and fire-bomber from Delta, BC.  John spoke at a meeting of the Langley Aviation Council.  He gave a fascinating presentation on the job of aerial bombing of forest fires, which I couldn’t possibly do justice to by reviewing.  However, at the end there was a discussion in which John passed on quite a few tips and rules of thumb that I thought readers of WCR would find interesting and useful.  Some of them are reproduced here, with John’s permission.  For a much more complete explanation of these and other flying tips, read John’s book, “IFR Hints and Pilot Principles.”
There’s an old saying:  a little knowledge is a dangerous thing.  I suppose that’s true, if the “little knowledge” tempts you to mess with something you don’t understand.  But, in flying, a little knowledge can sometimes save the day.  Here are some handy rules of thumb and simple calculations that can make flying easier, and safer.

Take-off performance:  altitude, winds, and slope

We all know that density altitude affects take-off performance.  So do winds and runway slope.  But by how much, and how do we allow for it?  Certified aircraft come with extensive charts from which you can calculate precise take-off distances.  If you’re really keen, you may have developed such charts for your RV.  But you can make reasonable allowances for density altitude, winds, and slope without the charts.

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.
 
 

Conditions

Field Elevation  4060 ASL
OAT  24 °C
Headwind Component  10 kt
Runway Slope  2.15 %

Performance

Estimated Density Altitude  5950 ASL
Take-off Roll:
  • Sea Level 
  • at density altitude 
  • with headwind 
  • with headwind & slope 
  • 550 ft
  • 880 ft
  • 790 ft
  • 1040 ft
  • 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.

    Climb Rate and Climb Gradient

    One of the easiest in-your-head calculations that can really help you out is the conversion between climb rate and climb gradient.
    To get feet per mile from feet per minute, divide by your true airspeed in miles per minute.

    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/minute
     = 750 feet per mile
    1,200 fpm @ 82/60 miles/minute
     ~ 900 feet per mile
    Actually, 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.

    The 1:60 Rule

    One of the reasons I like using nautical miles, rather than statute miles (or kilometres), is the 1:60 rule.  The 1:60 rule is based on the mathematical ratio of the length of an arc to its radius.  The length of an arc is equal to its radius when the angle of the arc is about 60 degrees.  (It’s actually equal at about 57.3 degrees, but 60 is close enough for rule-of-thumb purposes).

    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!

    Thunderstorms

    There are few things that worry pilots more than thunderstorms.  We all want to know how to avoid them.

    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 clear.
     
     

    Performance Rules of Thumb

  • For density altitude, add 1,000 feet to the field elevation for each 8°C above standard temperature.
  • Increase sea-level take-off distance by 10% for each 1,000 feet of density altitude.
  • Decrease take-off distance by 1% per knot of headwind; increase take-off distance by 10% for each 2 knots of tailwind.
  • Triple the runway gradient (in percent) and treat that like knots of wind (headwind if downslope and tailwind if up-slope).

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    Van's Air Force Western Canada Wing is not affiliated in any way with Van's Aircraft Incorporated. Western Canada RVator is not a publication of Van's Aircraft or any other corporation. All products reviewed or mentioned are not necessarily recommended for use by RV builders, but are described for information only. All builder's tips are presented only as a source of information and a forum for exchange and the sharing of ideas and construction methods. No responsibility is assumed, expressed, or implied as to the suitability, accuracy, safety, or approval thereof. Any party using the suggestions, ideas, or examples does so at his or her own risk and discretion and without recourse against anyone. The members of Van's Air Force Western Canada Wing, the editor of the Western Canada RVator, and all authors and contributors are not responsible for any product or builder's tips misuse, incorrect construction, or design failure, nor any other peril.

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