How do you find best glide speed in practice?

Best glide speed is typically provided in the aircraft's flight manual and corresponds to the minimum drag speed (VMD); it can also be identified as the lowest point on the total drag curve.

Why might a pilot choose to fly slightly above best glide speed?

Flying slightly above best glide speed improves speed stability and reduces pilot workload, making it easier to maintain control, especially in emergencies.

Does best glide speed change with aircraft weight?

Yes, best glide speed increases with weight, but the glide angle and maximum range remain the same if the correct speed is flown.

How do winds affect best glide performance?

A headwind reduces ground distance covered, so a higher airspeed is used; a tailwind increases ground distance, so a slightly lower speed may be optimal.

Best Glide Speed

Q: What is best glide speed?

Best glide speed is the airspeed at which an aircraft achieves the maximum lift-to-drag ratio, resulting in the greatest horizontal distance covered per unit of altitude lost.

Medium4 min readPrinciple of Flight

Moderately Examined

Why this matters

Knowing and using best glide speed is critical for maximizing safety and landing options during engine failure or forced landings. It helps pilots make informed decisions to reach a safe landing area and manage aircraft control under pressure.

Best glide speed is the airspeed that gives an aircraft the greatest horizontal distance per unit of altitude lost in a power-off glide. It is found at the speed where the lift-to-drag ratio is maximized, corresponding to the minimum drag point on the total drag curve. This speed is crucial for maximizing glide range after engine failure or when planning a forced landing.

Quick Check

At what speed is the lift-to-drag ratio maximized during a glide?

AI Tutor

Go beyond the textbook.

Ask Avi AI about Best Glide Speed

In depth

Explanation

What is Best Glide Speed?

Best glide speed, often referred to as V_MD (minimum drag speed), is the speed at which an aircraft achieves the best lift-to-drag (L/D) ratio. Flying at this speed during a glide results in the shallowest descent angle, allowing the aircraft to cover the maximum horizontal distance for every foot of altitude lost.

Total Drag Curve and Glide Performance

The total drag curve shows how drag varies with airspeed. At V_MD, total drag is at its lowest, and the L/D ratio is at its highest. Flying slower than V_MD increases drag due to higher induced drag, while flying faster increases drag due to higher parasite drag. Both cases reduce glide range.

Best Glide Ratio and Practical Adjustments

While the theoretical best glide speed maximizes range in still air, real-world factors like speed stability and workload come into play. Slightly increasing speed above V_MD improves speed stability, making it easier for the pilot to maintain a steady glide, especially under stress or turbulence. This small sacrifice in glide angle is often outweighed by improved control and decision-making.

Effects of Wind and Weight

Headwind: To maximize ground distance in a headwind, increase glide speed above V_MD. This compensates for reduced groundspeed.
Tailwind: In a tailwind, a slightly lower speed than V_MD can be used to take advantage of the increased groundspeed.
Weight: Heavier aircraft must glide at a higher indicated airspeed to maintain the same glide angle, but the maximum glide range remains unchanged. However, the rate of descent increases.

Minimum Rate of Descent (V_MP)

If the goal is to stay airborne as long as possible (endurance), fly at the speed for minimum power required (V_MP), which is slightly slower than V_MD. This minimizes sink rate but does not maximize range.

CG Location and Stability

A forward center of gravity (CG) increases stability but may require a slightly higher glide speed. An aft CG reduces stability and may make it harder to maintain the correct glide speed.

How to Find Best Glide Speed

Best glide speed is typically published in the aircraft's flight manual. It is determined during flight testing and is usually given as indicated airspeed (IAS), which remains valid regardless of altitude, though true airspeed (TAS) increases with altitude.

The essentials

Key Points

Best glide speed (V_MD) gives maximum range in a glide by maximizing the lift-to-drag ratio.

It is found at the bottom of the total drag curve.

Flying faster or slower than V_MD reduces glide range.

Headwinds require a higher glide speed to maximize ground distance; tailwinds allow a slightly lower speed.

Aircraft weight changes the best glide speed (higher weight = higher speed), but not the glide angle.

For maximum endurance (minimum sink rate), use V_MP, which is slower than V_MD.

Speed stability is better slightly above V_MD, which helps in real emergencies.

Watch out

Exam Traps & Typical Mistakes

Confusing best glide speed (range) with minimum sink speed (endurance).

Assuming best glide speed changes the glide angle with weight—glide angle stays the same if V_MD is adjusted for weight.

Forgetting to adjust glide speed in headwind or tailwind conditions.

Believing that flying slower than V_MD improves glide range—it actually increases drag and steepens the descent.

Not recognizing that best glide speed is an indicated airspeed, not true airspeed.

Test yourself

Example Exam Questions

Question 2Medium

If you encounter a strong headwind during a glide, how should you adjust your speed to maximize ground distance?

Question 3Medium

How does increasing aircraft weight affect the best glide speed and glide angle?

Still not fully confident?

Deepen your knowledge with an AI tutor built specifically for EASA ATPL students.

Built from thousands of ATPL knowledge references, real exam references and official learning objectives.

Open Avi AI Tutor

Keep going

Related Concepts

Still have questions?

Ask questions in plain English and get exam-focused explanations from an AI tutor built specifically for EASA ATPL students.

Open Avi AI