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Considering wide body aircraft, why do emergencies, related to hydraulic system failure, increase the landing distance?

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Landing distance is a function of the formula

Kinetic Energy = 1/2 * mass * velocity²

Any change that will increase kinetic energy will require longer landing distances. When we land, the kinetic energy needs to go somewhere. It can be converted into heat with the brakes, opposed with drag from the thrust reversers or deployed spoilers

  1. Higher landing speed will cause an increase kinetic energy that must be converted on landing and thus you will have a longer landing distance.
  2. Braking, thrust reverser or spoiler issues will increase the landing distance because they will not be as effective (take more time) in converting kinetic energy.

In a hydraulic issue, most likely the manufacturer has chosen to fly the airplane at a speed greater than needed for a normal landing (VREF + 20) and will most likely not have the ability to use normal braking. Most jets use hydraulic fluid to stop the brakes.

In the jets I fly, without normal braking means we use the emergency brakes which are not routed through the anti-skid system so precaution must be taken to not skid the tires. This all adds up to a substantial landing distance penalty. Hare the landing distances for some airplanes.

  • Learjet family: Factor of 3.15
  • Learjet 45 family: Factor of 2.2
  • Challenger 604 family: Factor of 1.7 (braking) times 1.8 (airspeed) for a total of 3.06
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Any hydraulic failure will affect the landing distance as most of aircraft controls are operated by hydraulics.

  • If the high lift devices (flaps) become inoperative (or become degraded) due to hydraulic failure, the aircraft will have a higher approach speed, which will increase the landing distance.

  • Hydraulics are also used for operating a number of other systems like brakes, ground spoilers and thrust reversers. Any degradation in their operation will increase landing distance.

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In most wide body aircraft hydraulics are used to lower flaps, raise spoilers, and in some cases apply wheel brakes. A flaps up landing is going to mean a higher landing speed, no spoilers will mean less drag to slow the aircraft, and not having wheel brakes will mean a much longer roll-out.

Also worth noting is that a hydraulic failure which will impact those systems will probably impact flight control surfaces as well, and landing gear extension, so a longer landing distance is the least of a pilot's worries in that situation.

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