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You’re heading from Taos, New Mexico to Los Alamos, just on the other side of the mountains, for a $100 taco at Dos Colores in your Cessna 172. While enroute, you’ve climbed to 13,500 feet for some sightseeing. Ten miles out over Cerro Peak, drilling along, fat, dumb and happy, the iron wind maker suddenly coughs and sputters. What to do, what to do?
Knowing the maximum glide speed of your trusty steed, you immediately go through your emergency actions and slow to 65 KIAS. This gives you a glide ratio of nine miles for each 6,080 feet of altitude. Can you put the airplane down somewhere away from the mountains? There is a mesa approximately 21 miles to the west that has an elevation of roughly 6,500 feet and you decide to try for it. At 13,500 feet, you should be able to glide about 20 nautical miles. However, it’s never that easy. You are actually 6,500 feet above the mesa and can glide only 9.5 miles. You would have to be at 21,000 feet (6,500 foot mesa plus airplane altitude of 13,500 feet above ground) to make it to the mesa. At your current altitude, to land on a piece of dirt with an elevation of 6,500 feet, you must pick a spot inside 10 miles of your location. All is not lost. If wind is no factor you could turn to the north and possibly make it back to the Taos golf course, making the most expensive hole-in-one you’ll ever experience.
Does this mean you have to be on an IFR flight plan, pressurized, or breathing oxygen to go sightseeing in mountainous terrain? Not if you plan what you are doing, instead of just mounting up and going for a ride (this isn’t a horse you’re maneuvering). Always have a Plan B that gets you out of trouble if you suddenly have an emergency. Always think about altitude, distance, and emergency landing areas as you are cruising along. An old rule of thumb is that any suitable landing zone 45 degrees, or more, below your line of sight, at any altitude, could be used for an emergency landing.
By “zooming” your aircraft to gain altitude until you slow down to maximum glide speed and then begin your descent you might be able to get a little more glide distance. That may make a significant difference if you have to perform a 180-degree turn to head towards your landing site. Try it on your next flight and make a note of how many feet you have to lead your level off so that you don’t go under your maximum glide speed.
Flying at the Pilot’s Operating Handbook (POH) maximum glide speed of 65 knots, a forced landing on the Taos golf course is imminent. You recognize you’re about to land on a beautiful green with a foursome ready to putt. You’re an avid golfer yourself and don’t want to shock them. What to do? Beep the horn, or pull up to stretch the glide? Regrettably, you can’t do either! You are already at maximum glide.
What is maximum glide? Simply put, it’s how far an aircraft can glide (power off) while covering the maximum distance across the ground. Or, how far forward the aircraft travels for each foot of altitude lost; expressed as Glide Ratio. Glide ratio is determined by dividing the distance covered at a particular airspeed by the rate of descent over a one-minute period.
In a trusty Cessna 172 I performed an informal flight test to determine its glide ratio, yielding the following data:
| KIAS |
FT/MIN*
|
RATE OF DESCENT
|
GLIDE RATIO
|
|
55
|
5573
|
475
|
11.7
|
|
60
|
6080
|
500
|
12.2
|
|
65
|
6586
|
550
|
12.0
|
|
70
|
7093
|
800
|
8.9
|
|
75
|
7600
|
900
|
8.4
|
| *Distance covered, in feet per minute, based on the KIAS shown to the left. |
Based on this flight data it appears 60 Knots Indicated Airspeed provides the best glide ratio. But, the Cessna POH tells us that 65 KIAS is maximum glide at maximum gross weight and provides a 9:1 glide ratio. What’s going on here? When performing this informal test, the engine was at idle with the propeller still turning. That rotating food processor out front creates more drag than one not turning, about 15-20 percent more. And, I am the first to acknowledge that I’m just a dumb fighter pilot, not a stupid fighter pilot. They pay real test pilots to stop the prop in flight.
Another reason the glide ratio differed from the POH is the airplane wasn’t at maximum gross weight. If you recall another old rule of thumb, for each 10 percent reduction in gross weight, reduce your glide speed by five percent. Additionally, there could be “stuff” in the pitot tube; the airspeed indicator itself could be a bit off; and all those neat new toys added to the airplane which have attendant antennae bolted to the wings and fuselage have an impact on the airplane’s performance. The factory airplane used to develop handbook data was much “cleaner.” And flown by aforementioned test pilots, I might add.
But, what’s important is this informal test’s results shows that tweaking your airspeed by pulling up the airplane’s nose, or pushing it down, affects glide ratio notably and not to your profit. This indicates, to me anyway, that staying at the airplane’s POH maximum glide airspeed in an engine-out emergency is the way to go.
Test your own aerospace vehicle to see how it performs, and how close the gauges are to tolerances. If you want to use a chart similar to the one above for testing your airplane, email me at JimVN@aol.com and put the words “Glide Schedule” in the subject line. I’ll send you an Excel version. Just tell me what version you need. Bear in mind, I don’t make any claims of precision or any legal yadda-yadda about it. It’s just a tool I use.
Knowing and using the appropriate maximum glide speed for your aircraft, and for the situation, will keep you out of trouble in an emergency. Now if you could only find the horn button to move those golfers!
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