Arresting Gear: Stopping a Jet in 100 Meters With a Wire

The catch: hook, cross-deck pendant, and the path below deck

A carrier landing is deliberately flown as a controlled crash: the pilot holds the glideslope and touches down at full throttle (so a miss can be flown away instantly) with no flare and no attempt to grease it on. Trailing from the aft fuselage is the tailhook — a hinged arm ending in a hardened, replaceable hook point. As the wheels slam down, the hook drags along the deck and, if the geometry is right, scoops up one of the arresting wires stretched across the landing area.

Each wire — properly a cross-deck pendant — is a length of flexible steel wire rope, about 35 mm (1-3⁄8 inch) in diameter, held roughly 5 inches above the deck by leaf-spring 'bow' supports so the hook can slide under it. Nimitz-class carriers historically strung four pendants spaced about 12 m apart; most now use three. Pilots aim for the 3-wire: the 1-wire sits dangerously close to the ramp (the stern edge), the 4-wire means a long, floated approach.

The pendant is only the visible end. Each end of the cross-deck pendant is joined by a cable terminal to a long purchase cable that runs outboard to a deck-edge sheave, turns 90° downward, and disappears below the flight deck to the arresting engine — the machine that actually absorbs the energy. When the hook grabs, both purchase cables are yanked out of the engine simultaneously, and everything the engine does to resist that pull is what stops the jet.

The physics: why a constant retarding force is the whole game

Stopping the aircraft is an exercise in the work–energy theorem. The engine must do work equal to the aircraft's kinetic energy over the available runout distance:

F · d = ½ m v²

Take a maximum-weight trap: m ≈ 22,700 kg (~50,000 lb) at v ≈ 67 m/s (~130 kn). The kinetic energy is ½ · 22,700 · 67² ≈ 51 MJ. Spread that over a 100 m runout and the average retarding force is F = 51×10⁶ / 100 ≈ 510 kN — over 50 tonnes of pull. Dissipated over the ~2–3 seconds of the runout, that is an average power on the order of 20 MW; because power is F·v and the force is nearly constant, the instantaneous peak comes at engagement and is closer to 34 MW, decaying to zero as the jet stops. All of it ends up as heat.

The deceleration follows from a = v² / (2d): for 67 m/s over 100 m, a ≈ 22.4 m/s² ≈ 2.3 g, and the stopping time t = 2d/v ≈ 3 s. Because the real engagement ramps force up quickly and then holds it, most traps feel like a hard two-to-three-second yank at 2–3 g, with brief peaks higher.

The crucial design insight is that a rectangular force profile — constant force over the whole runout — is optimal. For a fixed energy and a fixed available distance, constant force gives the lowest possible peak deceleration. Any force that spikes and then sags (or vice versa) either overloads the airframe early or runs out of deck before the aircraft stops. Every arresting engine is therefore an attempt to make the retarding force flat versus distance, independent of how heavy or fast the aircraft is.

The hydraulic engine and the cam-profiled orifice

The classic answer is the U.S. Navy MK 7 arresting engine, a hydro-pneumatic machine. The purchase cables are reeved in a block-and-tackle fashion around a bank of fixed sheaves and a movable crosshead attached to a large hydraulic ram. Because the cable is reeved through many falls (a reeving ratio of roughly 1:12–1:16), the ~100 m of cable that pays out moves the ram less than ten meters — trading a long, low-tension cable pull for a short, very high-force ram stroke.

As the ram is dragged out, it displaces hydraulic fluid (an ethylene-glycol/water mix) through a control valve into an air-charged accumulator, pressurizing it to thousands of psi. The retarding force on the aircraft is set by the fluid pressure acting on the ram area. Here is the elegant part. For turbulent flow through an orifice, the pressure drop scales with the square of flow rate:

ΔP = ρQ² / (2 C_d² A²) ,   Q = C_d A √(2ΔP/ρ)

Cable-out velocity — and therefore ram velocity and flow rate Q — starts high and falls to zero as the aircraft slows. If the orifice area A were fixed, ΔP would fall as v², the force would collapse, and the runout would be far too long. To hold the force constant as the flow dies away, the orifice area must shrink along the stroke. That is exactly what the constant-runout valve (CROV) does: a cam-driven tapered plunger, its profile machined so that closing the orifice at just the right rate keeps ΔP — and thus the retarding force — nearly flat from touchdown to stop. Before each recovery the arresting-gear crew dials a weight setting into the engine for the expected aircraft, which biases the cam so heavy jets meet more force and light jets less, keeping the runout distance roughly constant either way. Deck-edge sheave dampers soften the initial shock when the slack wire is suddenly snatched taut.

The rope geometry: why the wire tension dwarfs the drag force

When the hook first grabs the pendant, the wire is nearly straight across the deck, so pulling it back into a shallow 'V' generates enormous cable tension for a modest rearward force. If each leg of the V makes an angle θ with the deck's athwartships line at the hook, the retarding force on the aircraft is F = 2T sinθ, where T is the tension in each leg. Early in the runout, when θ is small (the V is shallow), the tension T = F / (2 sinθ) spikes to values well above the ~510 kN drag force — this is the moment the pendant and its splices are most highly stressed. As the hook pulls the wire into a deeper V, θ grows and the required tension drops.

This is the same vector statics that governs any loaded cable turning over a sheave: tension, included angle, and the resultant load are geometrically coupled (rope-on-sheave friction — the capstan effect — is a second-order correction layered on top), and the wire rope must be sized for the peak tension, not the average pull. It is also why off-center engagements are dangerous — if the hook catches the wire well to one side, the two legs carry unequal tension and load asymmetrically, and the aircraft yaws as it decelerates.

Advanced Arresting Gear: water turbines and closed-loop control

The Gerald R. Ford class (CVN-78) replaced the hydro-pneumatic engine with General Atomics' Advanced Arresting Gear (AAG). Instead of a ram and orifice, AAG dissipates the bulk of the energy in a rotary 'water twister' — a paddle-wheel spinning in a water-filled housing, where turbulent viscous drag on the paddles converts the cable's pull into heat — in effect a water-brake dynamometer, the same principle as a heavy truck's hydraulic retarder. In parallel, an induction motor coupled to the drivetrain gives the system what the passive valve never could: active, closed-loop control.

Because the motor can add or subtract torque under digital control and read the cable payout electronically, AAG can shape the retarding-force profile in real time to match the specific aircraft, keeping peak airframe loads lower and more repeatable, and — critically — it can arrest a much wider mass range, from heavy fighters down to light unmanned aircraft the hydraulic MK 7 could not safely catch. The motor also drives the system in reverse to retension and retract the pendant between traps. AAG's development was famously troubled — long delays, cost overruns, and mean-cycles-between-failure well below target during land-based testing at Lakehurst — but it embodies the trend from a cleverly-shaped mechanical cam to a software-defined force profile.

Materials, fatigue, and how it is tested

The cross-deck pendant lives a brutal life: every trap subjects it to a peak tension of hundreds of kN, sharp bending over the deck-edge sheaves, and abrasion from steel hook points. It is high-carbon galvanized steel wire rope, typically a flexible 6×30-class construction chosen for bending endurance, with a breaking strength on the order of 200,000 lb (~900 kN). Even so, fatigue and wear govern its life: a pendant is inspected after every recovery and retired after roughly 100–125 traps, well before broken wires or a worn cross-section can trigger a failure. The tailhook point is a forged, heat-treated high-strength alloy steel (of the 4340 class), designed to be swapped out as it wears from repeated hammering on the deck.

Engines and cables are qualified on land-based test sites — the Navy's runway-installed engines and dead-load 'jet car' tracks at Lakehurst, New Jersey — where instrumented dead loads are fired down a track into the gear at controlled weights and speeds while load cells and high-speed cameras record cable tension, runout, and peak deceleration. The acceptance criterion is that, across the full weight–speed envelope, the runout stays within the deck and the peak airframe load stays under the aircraft's hook-load limit with margin. Land-based friction-brake systems like the USAF BAK-12 are qualified the same way against MIL-standard runway-arrestment requirements.

How it fails, and the land-based cousins

The most common non-catch is the bolter: the hook skips over or misses all the wires. Because the pilot lands at full power, a bolter is a non-event — the jet simply flies off the angled deck and comes around for another try. That angled deck exists precisely so a bolter or missed wire has clear runway ahead instead of parked aircraft. Other failure modes are more serious:

• Hook skip / hook bounce — the hook point bounces off the deck and hops over a wire, turning an intended trap into an unplanned bolter.
• In-flight engagement (IFE) — the hook catches a wire before the wheels are down, so the full arrest load hits the airframe through the hook alone at an odd attitude.
• Cable failure and whiplash — if a worn pendant or purchase cable parts under peak tension, the stored elastic energy releases as a lethal whiplash across the deck; real incidents — such as the March 2016 cable failure aboard USS Dwight D. Eisenhower, which injured eight sailors while the landing E-2C staggered off the bow and managed to fly away — have swept crew off their feet and left the aircraft with no arrestment at all. This is why cable retirement is conservative and deck personnel stand well clear of the run.

When the hook or landing gear is damaged, the ship rigs the barricade — a nylon-webbing net that engages the aircraft's wings and feeds the same MK 7 engine. On land, runway-overrun protection uses the same principle with different absorbers: the BAK-12 rotary-friction system and net barriers (BAK-15), and mobile aircraft arresting systems catch an aborting or brakeless jet at a runway's end, dissipating its energy in a friction brake or a soft-ground EMAS arrestor bed. The unifying idea across all of them is the one the carrier engine perfected: apply a controlled, near-constant force over the longest distance available, and turn a survivable amount of the aircraft's kinetic energy into heat.

Arresting systems compared — each converts the same kinetic energy to heat, but through a different absorber and control scheme.

System	Energy absorber	Force control	Example / limits
MK 7 Mod 3 (hydro-pneumatic)	Hydraulic ram forces fluid through a valve into an air accumulator	Cam-profiled constant-runout valve — passive, weight-set	Nimitz-class carriers; ~50,000 lb at ~130 kn
Advanced Arresting Gear (AAG)	Rotary 'water twister' paddle turbine in a water-filled housing	Induction motor + closed-loop digital control — active, tunable	Ford-class (CVN-78); wider aircraft envelope incl. UAVs
BAK-12 (land-based)	Rotary friction brake on a cable-storage drum (tape/cable)	Preset friction torque vs. runout	USAF runway overruns; ~50,000 lb, up to ~180 kn
Barricade (emergency)	Nylon webbing net feeds the same MK 7 engine	Same engine, engages wings not hook	Used when the hook or gear is damaged