Plan a descent for a 185-knot speed restriction with practical distance rules, a worked example, FMC checks and fixes for excess energy.
To meet a 185-knot descent restriction, work backwards from the restricted fix: identify the required altitude and indicated airspeed, calculate descent distance, add a separate deceleration allowance, then add wind and workload margin. Begin slowing early enough to be stabilised at 185 KIAS before crossing the fix—not merely selecting 185 there.
For Aviation & Real-World Flying, read 185 knots as indicated airspeed unless the chart or clearance states otherwise. Ground speed affects how much distance you need, but it does not determine compliance. Our explanation of IAS, Mach and their use during descent covers the high-altitude transition.
What does a 185-knot restriction actually require?
The exact wording determines when the aircraft must reach 185 knots and whether that figure is a target or a maximum.
| Instruction or notation | Required action |
|---|---|
| Cross the fix at 185 knots | Reach approximately 185 KIAS before the fix and cross it at that speed. |
| Cross at 185 knots or less / MAX 185 | Cross at or below 185 KIAS, subject to any minimum safe or procedural speed. |
| Reduce speed to 185 knots | Begin a prompt reduction and maintain 185 until another clearance or restriction applies. |
| Maintain 185 knots until the fix | Do not slow below 185 before crossing unless ATC changes the instruction. |
Published procedure notation and FMC coding conventions vary, so check the chart legend rather than assuming that every bare speed entry has the same meaning. A 185-knot restriction is also separate from flap-limit speeds, manoeuvring speeds and landing V-speeds.
How far before the fix should I start slowing?
Start far enough back to complete both the required altitude loss and the speed reduction without relying on a steep descent, maximum drag or a last-minute level-off.
For an initial three-degree descent estimate, use:
Descent distance (NM) ≈ altitude to lose (ft) ÷ 1,000 × 3
Calculate the speed reduction separately:
Speed to lose = present IAS − 185
The aircraft's FMC, performance data and operating procedures are the proper sources for deceleration performance. When learning in a simulator without dependable performance data, a useful starting estimate is roughly 1 NM per 10 knots in level flight. Allow closer to 2 NM per 10 knots while descending clean, because gravity is helping the aircraft retain speed. These are planning rules, not certified performance figures.
A conservative backwards plan therefore uses:
Total distance ≈ descent distance + deceleration distance + margin
The FMC may blend descent and deceleration into a shorter path, but treating them separately helps prevent the familiar high-and-fast arrival. For broader energy and top-of-descent planning, see our method for planning and flying a jet descent.
Worked example: 240 knots to 185 knots
Suppose the aircraft is at 8,000 feet and must cross the fix at 3,000 feet and 185 KIAS. The altitude loss is 5,000 feet, giving about 15 NM on a three-degree path.
The required speed loss is 55 knots. That needs about 6 NM using the level-flight estimate, or approximately 11 NM if the aircraft must slow while continuing downhill. A conservative plan would therefore begin around 21–26 NM before the fix, plus extra margin for a tailwind, high mass, anti-ice operation, turbulence or unfamiliar aircraft behaviour.
Only descend when cleared or authorised by the published procedure. Starting the calculation early does not authorise an early descent.
How should I fly the planned descent?
Fly the descent by controlling altitude and speed as two linked energy problems rather than expecting the selected speed or autothrottle to solve both automatically.
- Confirm the constraint. Check the fix, altitude, speed wording and whether 185 is exact, maximum or a maintain-until instruction.
- Work backwards from the fix. Reserve distance for deceleration, then calculate where the altitude descent must begin. Increase the allowance for a tailwind or an aircraft that is slow to shed energy.
- Enter and verify the FMC restriction. Put the speed and altitude against the correct waypoint, then inspect the resulting vertical path. Coding errors, route discontinuities and inaccurate wind forecasts can invalidate the prediction.
- Monitor actual energy. Check IAS trend, ground speed, vertical path and distance remaining. Selecting 185 commands a target; it does not create drag.
- Capture 185 before the fix. Unless instructed to maintain a higher speed until that point, aim to settle at 185 a few miles early instead of arriving fast and decelerating through the fix.
For the 737 specifically, our 737 descent and approach setup explains how top-of-descent, deceleration and terminal restrictions fit together.
What if the aircraft will not slow to 185 knots?
If the aircraft is not decelerating, reduce the descent angle or level temporarily when clearance and terrain permit; a clean jet usually cannot descend steeply and lose substantial speed at the same time.
- Verify that thrust has reduced as expected and that the active automation mode is actually commanding 185 knots.
- Trade descent rate for deceleration by using a shallower path. Do not continue diving towards the altitude constraint while hoping the speed will disappear.
- Use speed brakes within the aircraft's limitations and retract them at the point required by the operating procedure. Our guide to using aircraft speed brakes correctly covers the associated configuration traps.
- Deploy flaps only below their placard limits and according to the normal schedule. Do not treat 185 knots as an automatic flap-selection cue.
- If compliance is no longer possible in real-world flying, tell ATC early rather than crossing the fix fast or departing from the cleared altitude.
A tailwind is a common reason that an apparently adequate plan fails: the aircraft covers more ground during each second of deceleration. High weight, increased idle thrust with anti-ice, a steep path and late flap deployment can compound the problem. Approved aircraft data, operator procedures and ATC instructions always take precedence over generic distance rules.