What Is the Glide Ratio of a Plane?
The glide ratio of an aircraft is the ratio of horizontal distance traveled to altitude lost during unpowered flight. A commercial airliner has a glide ratio of approximately 17:1 to 20:1 — meaning for every foot it descends, it travels 17 to 20 feet horizontally. At cruising altitude (35,000 feet), a fully unpowered airliner can glide approximately 100 miles (160 km) before needing to land. This is a fundamental safety characteristic: even with complete engine failure at altitude, the aircraft does not fall — it glides.
For a complete picture of how aircraft generate lift: How Does a Plane Fly?. For the role of airspeed in controlling glide performance: What Is the Speed of a Commercial Airplane?.
What Is Glide Ratio?
Glide ratio (also called lift-to-drag ratio) measures aerodynamic efficiency in unpowered flight. An aircraft with a glide ratio of 18:1 can travel 18 units horizontally for every 1 unit of altitude it loses. A glider (sailplane) can have a glide ratio of 50:1 or higher; a paper airplane is around 4:1; a typical skydiver in freefall is about 1:1 (straight down).
A commercial airliner's glide ratio is determined primarily by its aerodynamic design — wing shape and area, fuselage drag, and the drag of the aircraft with engines at idle (engines off create less drag than powered engines). Most current commercial jets have best glide ratios between 15:1 and 22:1.
What Happens in an Engine Failure?
Complete engine failure in commercial aviation is extraordinarily rare. But when it does occur — or in the extremely unusual case of complete power loss — the aircraft does not immediately descend steeply. The wings continue to generate lift as long as the aircraft maintains flying speed.
From cruising altitude of 35,000 feet, a Boeing 737 or Airbus A320 with no engine power has approximately 22 minutes of glide time before reaching an altitude suitable for landing — and a radius of action of roughly 100 miles. Air traffic control would have ample time to vector the aircraft to the nearest suitable airport. Pilots would run through engine restart procedures, which are frequently successful.
The Gimli Glider: a real-world example
The most famous example of a successful unpowered commercial landing is Air Canada Flight 143, known as the 'Gimli Glider.' In 1983, a Boeing 767 ran completely out of fuel at 41,000 feet over Manitoba due to a fueling calculation error. The crew successfully glided the aircraft and landed at a former air force base at Gimli, Manitoba — with no fatalities. The aircraft was repaired and returned to service.
US Airways Flight 1549: engines, not glide
Often cited as a 'glide,' the 2009 Hudson River landing of US Airways 1549 was actually a powered approach at reduced power — both engines were severely damaged (not fully off) and the aircraft had limited thrust available. Nevertheless, the crew's ability to manage a complex emergency and land precisely on the river is a testament to training and aircraft handling.
How Glide Performance Is Managed
Pilots train regularly for engine failures at all phases of flight. The procedures are memorized and practiced: secure the failed engine, start the APU (auxiliary power unit) for electrical power, attempt restart, declare emergency if needed, select diversion airport.
Modern flight management computers instantly calculate optimal glide speed, range to airports, and fuel state. In a genuine emergency, the cockpit has tools that a layperson would not expect — the crew is never simply hoping the aircraft will stay aloft.
The Reassurance in the Numbers
For anxious flyers, the glide ratio is one of the most reassuring facts in aviation. The aircraft you're on is not defying gravity — it is continuously managing it with precision. Even in the most extreme hypothetical scenario, the aircraft does not fall: it glides. For further context on the physics: Why Do Planes Take Off and Land Into the Wind?.
Sources
NASA's Glenn Research Center explains glide ratio and lift-to-drag ratio in accessible educational language: NASA: Glide ratio and flight efficiency.
Popular Mechanics covers what actually happens when an aircraft loses engine power: Popular Mechanics: Can a Plane Glide Without Engines?.
The Smithsonian Air and Space resources cover aerodynamics fundamentals: Air & Space Magazine: The Science of Engineless Flight.
FAQ
Can an airliner land without engine power?
Yes. This has been demonstrated in real-world incidents including the Gimli Glider. Modern airliners are designed with backup hydraulic and electrical systems that do not require engines to operate. The aircraft can be controlled and landed without power — it is more difficult and requires a suitable runway, but it is achievable.
What is the minimum speed for gliding?
The best glide speed (Vbg) for most commercial jets is in the range of 220–280 knots, depending on aircraft weight. Below this speed, the aircraft descends more steeply; above it, extra drag reduces glide efficiency. Pilots select best glide speed from memory as one of the first actions after an engine failure.
Does the glide ratio change with aircraft weight?
Glide ratio itself doesn't change significantly with weight. What does change is the airspeed needed to achieve the best glide ratio: heavier aircraft need higher speeds. Distance covered in a glide remains similar regardless of weight.
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