When an engine fails on a jet transport at cruise altitude, two performance questions become immediately critical: how far can the aircraft glide if all thrust is lost, and how does it descend to a survivable single-engine altitude if one or more engines remain operating? The answers depend on precise speed management tied to aerodynamic fundamentals — specifically the concept of maximum lift-to-drag ratio and the regulatory driftdown profile. Understanding both concepts is essential for Airline Transport Pilot candidates and for any crewmember operating transport-category aircraft under 14 CFR Part 121 or Part 135.
These speeds are not arbitrary numbers assigned by manufacturers. They emerge directly from the aircraft's aerodynamic polar — the relationship between lift coefficient and drag coefficient across the speed range — and they are codified in the FAA-approved Airplane Flight Manual (AFM) for every transport-category aircraft. This article examines how each speed is derived, why it matters operationally, and the key numbers and traps that appear on knowledge tests and oral exams.
Maximum Range Glide Speed: The Aerodynamic Foundation
Glide performance is governed by the lift-to-drag (L/D) ratio. When an aircraft glides with engines producing no thrust, the forces acting on it are lift, drag, and weight. The glide angle is determined by the ratio of drag to lift: a shallow glide angle requires high L/D, meaning lots of lift produced for each unit of drag. The speed that produces the highest L/D ratio is called L/D max, and it is the speed that yields the greatest horizontal distance per unit of altitude lost — in other words, maximum glide range.
For a typical jet transport in the clean configuration, L/D max corresponds to a specific angle of attack, not a fixed airspeed. Because lift equals weight in a glide (approximately), and because lift is proportional to air density, the calibrated airspeed that produces L/D max increases with weight and decreases with altitude. Pilots must consult AFM charts or use a speed indexed to gross weight — some manufacturers express it as a function of current weight. The FAA-H-8083-25C notes that the best glide speed for maximum range is found at the point where induced drag equals parasite drag, which is the aerodynamic definition of L/D max.
It is important to distinguish maximum range glide from minimum sink rate glide. Minimum sink rate — which keeps the aircraft airborne for the longest time — occurs at a slightly lower speed than L/D max. Pilots who fly slower than L/D max in an attempt to stretch the glide will actually increase the glide angle and arrive at the ground sooner horizontally. This is a counterintuitive but testable fact. Maximum range is achieved only at L/D max, not by pulling the nose up further.
Effect of Wind on Glide Speed
In a headwind, pilots should increase glide speed above L/D max to compensate for the reduced ground speed; in a tailwind, the opposite applies — reduce slightly toward minimum sink speed to maximize time aloft and ground coverage. This optimization mirrors the logic applied to best range cruise speed adjustments for wind. However, the AFM-published glide speed is based on still-air conditions, and most emergency checklists do not offer a wind-adjusted table; in practice, the published AFM speed is used as the baseline.
Driftdown: Engine-Out Descent to Single-Engine Ceiling
Driftdown is the procedure used when one or more engines fail at cruise altitude and the remaining engine(s) cannot maintain that altitude. Unlike a pure glide (no thrust), driftdown involves residual thrust from the operating engine(s). The aircraft cannot climb or hold altitude, so it descends — or drifts down — while the crew manages speed and configuration to minimize altitude loss and maximize the final level-off altitude, known as the single-engine service ceiling.
The single-engine service ceiling is the maximum pressure altitude at which the aircraft can maintain a steady 50-foot-per-minute rate of climb on the remaining engine(s) at maximum continuous thrust (MCT), at a weight appropriate to the conditions. This ceiling is published in the AFM performance section and typically falls significantly below the aircraft's normal cruise altitude. For a twin-jet, losing one engine at, say, FL 390 may require driftdown to FL 220 or lower depending on weight and temperature.
The objective of the driftdown maneuver is to reach that single-engine service ceiling with the maximum possible residual altitude — meaning the driftdown profile should be flown as efficiently as possible. The optimal driftdown speed balances the thrust available from the remaining engine(s) against aerodynamic drag. Flying too fast increases parasite drag and wastes energy; flying too slow increases induced drag and may approach the buffet boundary. The FAA-H-8083-25C describes this as a speed that changes as weight decreases during the descent due to fuel burn, so AFM driftdown charts present an optimum speed schedule — typically decreasing calibrated airspeed as the aircraft descends and lightens.
Regulatory and Operational Framework
Under 14 CFR Part 121, air carriers operating in extended-range operations and in certain mountainous terrain must demonstrate compliance with obstacle clearance requirements along the planned route when engine-out driftdown is considered. This is commonly evaluated as part of the net flight path requirement — the actual engine-out flight path reduced by a specified gradient (typically 1.1% for two-engine airplanes at the en-route phase) to provide a margin of error. The net flight path must clear all terrain and obstacles by at least 2,000 feet vertically or 5 statute miles laterally along the planned route. Dispatchers and pilots must verify these criteria during preflight planning, especially for high-terrain routes such as trans-mountain crossings.
The drift-down speed is also relevant to ETOPS (Extended-range Twin-engine Operational Performance Standards) planning, where the ability to reach a suitable alternate airport on one engine determines authorized diversion time. Although driftdown speed is not referenced by name in the ETOPS rule itself, the underlying single-engine performance capability — validated through driftdown analysis — is central to ETOPS approval.
Key Numbers and Rules
- L/D max speed = maximum range glide speed. It occurs where induced drag equals parasite drag on the drag curve. Flying faster or slower than this speed reduces glide range.
- Minimum sink speed is slower than L/D max — it maximizes time aloft, not distance traveled.
- Glide ratio for modern jet transports typically ranges from about 15:1 to 20:1 in the clean configuration at L/D max. A ratio of 17:1 means 17 nautical miles of glide range per nautical mile of altitude lost — roughly 10 miles per 3,000 feet of altitude.
- Single-engine service ceiling is defined as the altitude where the aircraft can sustain 50 fpm climb on remaining engine(s) at MCT.
- Driftdown speed decreases as weight decreases during the descent — a variable speed schedule, not a fixed number.
- Net flight path terrain clearance: 2,000 feet vertical or 5 statute miles lateral clearance required by 14 CFR Part 121 en-route obstacle rules.
- Headwind adjustment: increase above L/D max; tailwind: consider reducing slightly toward minimum sink speed to maximize range over ground.
Why These Speeds Matter Operationally
Failing to fly the correct glide speed in a total engine failure can cost a jet transport crew hundreds of miles of glide range — potentially the difference between reaching a runway and landing in terrain. The 2001 Gimli Glider incident (Air Canada B767 fuel exhaustion) and the 2001 Azores Glider (Air Transat A330) both demonstrated that crews trained in best-glide speeds and procedures can successfully execute long deadstick approaches. Flying even 20 knots faster than L/D max on a large jet can reduce glide ratio by a measurable fraction, compounding over long descents from cruise altitude.
For driftdown, the stakes involve terrain clearance over mountain ranges. An operator who has not verified that the net engine-out flight path clears the Rocky Mountains or the Alps may be planning an unachievable route. This is why pre-departure performance planning — not just in-flight reaction — is where driftdown awareness pays off. Dispatchers and pilots share responsibility under 14 CFR §121.99 and §121.193 for ensuring engine-out en-route performance is adequate before departure.
Common Test Traps
- Confusing minimum sink with maximum range glide. Examinees sometimes believe the slowest safe speed maximizes glide range. It does not — L/D max, a slightly higher speed, maximizes range. Minimum sink maximizes time aloft.
- Treating glide speed as weight-independent. L/D max is an angle of attack, not a fixed IAS. A heavier aircraft reaches L/D max at a higher indicated airspeed. The AFM typically provides a weight-based table or formula.
- Assuming driftdown speed is constant throughout the descent. It decreases with weight as fuel burns off, so the proper technique follows a decreasing speed schedule from the charts.
- Forgetting the 50 fpm criterion for single-engine service ceiling. Students often confuse service ceiling (50 fpm residual climb) with absolute ceiling (0 fpm) or drift-down floor.
- Overlooking net flight path margins. The gross engine-out flight path is reduced by a regulatory gradient to derive the net flight path — terrain must be cleared by 2,000 feet vertically, not just by any margin. Using gross performance data for terrain clearance planning is a dangerous and testable error.