By Jim Van Namee
When the batteries fail, or you don’t own an electronic E6B or Garmin 1000 panel, you can still fly with precision using the old Mark 1, Mod 0 gray matter that lies between your headset. Listed below are some “Physics for Poets” techniques that may help you in the cockpit when you need to figure out what to do next. Temperature and humidity can affect the results, but they are close and usually adequate for inflight planning.
Temperature:
C to F: Centigrade Temp x 2 less 10% + 32 = Fahrenheit
F to C: Fahrenheit Temp – 32 + 10% divided by 2 = Centigrade
True Airspeed:
Indicated Altitude x 2% per 1000’ above Sea Level + Indicated Airspeed
E.G. – 8000’ MSL at 125 KIAS
8 x 2 = 16%
125 x 1.16 = 145 KTAS
Standard Rate Turn Bank Angle
Method 1:
Airspeed – Remove last digit and add 5
E.G. – 120 KIAS => 12 + 5 = 170 Bank Angle
90 KIAS => 9 + 5 = 140 Bank Angle
60 KIAS => 6 + 5 = 110 Bank Angle
Method 2:
(True Airspeed/10) x 1.5
E.G. – 120 KIAS/10 = 12 x 1.5 = 180 Bank Angle
When to Begin Enroute Descent @ 500 FPM Descent:
60 Knots  Double your altitude difference by 2
E.G. – Leaving 11000’ MSL for 8000’ MSL
11000 – 8000 = 3000
3 x 2 = 6 NM Out – Begin your descent
120 Knots  Double your altitude difference by 2 then by 2 again
E.G. – Leaving 10000’ MSL for 8000’ MSL
10000 – 8000 = 2000
2 x 2 = 4 x 2 = 8 NM Out – Begin your descent
When descending to pattern altitude, add an additional 45 NM in order to be at pattern altitude prior to entering the pattern.
Reciprocal Headings:
When Primary Heading is greater than 1800:
Primary Heading  200 + 20 = Reciprocal Heading (PH of 270200=070+20=090)
When Primary Heading is less than 1800:
Primary Heading + 200  20 = Reciprocal Heading (PH of 130+200=33020=310)
Using Pitch Attitude to Maintain Rate of Descent:
Indicated Mach Number (IMN) x Pitch Attitude Change (PC) = Rate of Descent Required
First we must change the Laws of Physics and say that Mach 1 = 600 Knots
Therefore: Mach .1 = 60 Knots
Mach .15 = 90 Knots
Mach .2 = 120 Knots
If we want to descent at 500FPM @ 60 Knots (Descent Rate is 500FPM and IMN is .1):
At 60 Knots => .1 x X = 500FPM, or:
.1 x 5 = 500FPM, therefore the Pitch Change is 50
At 90 Knots => 1.5 x 3 = 450FPM, therefore the Pitch Change is 3.50+
At 120 Knots => 2 x 2.5 = 500FPM, therefore the Pitch Change is 2.5
This works flaps up or at approach setting, assuming you maintain airspeed.
Remember, it is Pitch CHANGE from level flight, not Pitch Attitude!
An ILS Rule of Thumb:
The faster you fly, the higher the rate of descent to stay on a 30 glide slope
You’re covering more ground, so you need a higher descent rate.
Therefore:
60 Knots on an ILS 30 glide slope requires a 323’/min rate of descent => 3.20 Pitch Change @ FAF
90 Knots on an ILS 30 glide slope requires a 484’/min rate of descent => 3.20 Pitch Change @ FAF
120 Knots on an ILS 30 glide slope requires a 646’/min rate of descent => 3.20 Pitch Change @ FAF
See any constants here?
When to Start a Descent for a Visual Straight In Approach:
Assuming a 500FPM descent it takes:
2 miles to descend 1000’ at 60 Knots ground speed;
3 miles to descend 1000’ at 90 Knots ground speed;
4 miles to descend 1000’ at 120 Knots ground speed.
Don’t forget to add .5 – 1.0 NM from your GPS derived distance as it computes distance to the airport based on the Airport Reference Point and not the end of the runway unless you are in an Approach Procedure Mode.
Take Offs or Landings on a Wet Runway = Hydroplaning Airspeed:
Hydroplaning begins at a speed equal to 9 x Square Root of your Tire Pressure.
If your main tires are inflated to 49 psi, then the square root of 49 is 7:
9 x 7 = 63 Knots Hydroplane Speed (Is this close to Vr or touchdown speed?)
Pressure Altitude (PA): Required for Density Altitude Computations
Method 1 – Go out to your a/c and set 29.92 in altimeter and read the PA, or
Method 2 – each .01” change of altimeter setting = 10 feet
If altimeter setting is lower than 29.92 then ADD the difference
If altimeter setting is higher than 29.92 then SUBTRACT the difference
E.G. – Altimeter setting is 30.52 and you are at a field elevation of 7100’ MSL
30.52 – 29.92 = .60 or 600’ => 7100 – 600 = 6500’ PA
Some Rules of Thumb:
Take Off
• T/O distance increases 15% for each 1000’ Density Altitude (DA) above sea level
• Available engine HP decreases 3% for each 1000’ DA above sea level
• Fixed pitch, nonturbo a/c climb performance decreases 8% for each 1000’ DA above sea level
• Variable pitch, nonturbo a/c climb performance decreases 7% for each 1000’ DA above sea level
• Vr = ~1.15 X Vs
Climb
• Vy decreases ~1/2 to 1 knot for each 1000’ DA
• Vy, Vx and Vg (best glide) decreases ~1/2 knot for each 100# under MGW
• Va decreases 1% for each 2% reduction in gross weight
Cruise
• TAS increases 2% over IAS for each 1000’ above sea level
• Cruise fuel consumption of a nonturbocharged a/c engine = ½ the rated HP/10
Approach
• Final approach speed = 1.3 X Vso. This is also known as Vref
• A 10 reduction in approach angle will increase landing distance 13%
• For each 1000’ increase in field elevation above sea level, stopping distance increases 4%
• 100 – 250 of flaps add more lift than drag; 250 – 400 flaps add more drag than lift
• A tailwind of 10% of your final approach speed increases your landing distance by 20%; A headwind of 10% decreases landing distance by 20%
• For each knot above POH Vref (final approach speed) over the runway end numbers, the touchdown point will be 100’ further down the runway
• A 10% change above POH recommended final approach airspeed will cause a 20% increase in stopping distance
• A slippery or wet runway may increase your landing distance by 50% or more.
Author’s Bio:
Jim Van Namee is a 6000+ hour CFII and FAA Aviation Safety Counselor. He is a retired Navy fighter pilot and owner of Silver Eagle Aviation, LLC in Taos, NM. Jim conducts the New Mexico Pilot’s Association Mountain Flying Clinic each year in the early fall.
