Safe Landing Technique: Insight into Thrust Reversal, Enhancing Aircraft Braking
How the hell do 400 tons of metal birds slamming down at over 150 miles an hour manage to grind to a halt? Let's unravel the mystery behind those death-defying landings, focusing on the role of engines in the post-landing slow-down.
Everyone's aware that brakes on the wheels help stop the aircraft, but let's dig deeper and explore how engines play a crucial part in the process. We'll cover the technique these aviation titans use to work in reverse and how you can spot the engines at work.
Key Takeaways
- Engines do more than just carry passengers -- reverse thrust helps slow the aircraft post-touchdown.
- Reverse thrust reduces brake wear, shortens landing distances, and improves safety during tough conditions.
- Different designs for reverse thrust include target (bucket), clamshell, and cascade systems.
- Proper usage is vital for safe landings and rejected takeoffs.
Unveiling Reverse Thrust
Defining Reverse Thrust
In aviation, "reverse thrust" harnesses the engines to slow the aircraft down by redirecting their thrust against its movement. Pilots don't mean spinning the engines backward or reversing the propellers' direction when referring to "reverse thrust." Instead, they concentrate on rerouting the airflow as it leaves the engines.
Various methods are employed to redirect exhaust gases or fan airflow, such as utilizing movable panels behind the engine or altering propeller blade pitch.
Executing Reverse Thrust
What gives with the conventional wheel brakes if engines can slow down the aircraft? Well, wheel brakes depend solely on the wheel's grip on the surface. If the runway's contaminated with water, ice, or oil, the wheel may skid over the surface instead of gripping it, leaving the brakes less effective. Engines, on the other hand, remember, are not irked by such surface conditions. As soon as the aircraft kisses the ground, reverse thrust can step in to lend a helping hand. This extra stopping power can be invaluable on short runways and during adverse weather.
Unmasking How Reverse Thrust Works
Cornerstone Concepts
Reverse thrust operates on a fundamental law of physics -- Newton's Third Law of Motion. This principle states that for every action, there's an equal and opposite reaction. Typically, engines throw exhaust gases or fan airflow backward, which pushes the aircraft forward. When we redirect the mass of air forward, the reaction pushes backward against the aircraft, slowing it down.
In reality, jet engine thrust reversers only redirect their exhaust by around 135 degrees instead of 180 degrees, far from being exactly opposite. And there's a good reason for that: practicality. It's a mouthful to fit all the necessary ducting, piping, and such to bend the exhaust 180 degrees. Plus, the jetblast might slam straight into the wings, flaps, and the runway, potentially causing damage or stalls.
Components in Motion
Since you've already got a powerhouse engine ready to go, you only need a system to redirect the exhaust.
- Most aircraft equip movable reverser doors, special panels that deflect the exhaust gases forward. These doors stow neatly when not in use to avoid hindering normal airflow. Some aircraft also employ additional blocker doors to obstruct the normal (backward) exhaust flow. Newer aircraft with low-bypass engines, like cascade systems, utilize vanes that guide the airflow forward.
- Actuators are the power behind these moving panels, with some using hydraulic, pneumatic, or electric systems. Propeller-driven aircraft handle reverse thrust differently, as we'll discuss later on.
Types of Reverse Thrust Systems
Target (Bucket) Reverse Thrust Systems
These systems, easy to spot when active, deploy two large bucket-like doors to build the smooth cone shape of the engine's exhaust nozzle. When pilots activate reverse, the buckets swing open, gasping for air to redirect their exhaust gases forward.
Target or bucket-style reverse thrusters were popular on many first-generation turbofan airliners and some military jets. For instance, early 737 models (737-100/200 with JT8D engines) had bucket reverse thrusters, which you could spot expanding on each engine during landing rollout. The Douglas DC-9 is another aircraft with the same type of reverse thrusters. These reverse thrusters typically operate hydraulically to ensure swift and secure deployment against the force of the exhaust.
Clamshell Reverse Thrust Systems
Clamshell systems shroud their doors inside the tailpipe, making them less conspicuous from the outside. When activated, the halves of the door swing inward like halves of a seashell. This blocks the nozzle throat and forces the exhaust to escape through slots or louvers just ahead of the doors. This design makes the system difficult to spot from the outside. Clamshell designs were common on earlier turbojet and low-bypass turbofan engines and are usually pneumatically or hydraulically actuated. They must withstand high temperatures as they handle the hot exhaust gases directly. An example of a BAC One-Eleven aircraft uses these kinds of reverse thrusters.
Cascade and Pivot Reverse Thrust Systems
The cascade and pivot type reverse thrusters are the most common types on modern aircraft. They cater especially to high-bypass engines.
Why do high-bypass engines need a different type of reverse thrust system? High-bypass turbofan engines have a large fan at the front, creating most of the engine's thrust.
Modern reverse thrusters take advantage of the fan's thrust by redirecting it rather than messing with the hot core exhaust, which is why they are sometimes called "cold-stream" reverse thrusters.
How Reverse Thrust Works in Propeller-Driven Aircraft
Most people disregard propeller aircraft when thinking of reverse thrust, but that's a misconception. It's true that propeller aircraft don't have additional components for a reverse thrust system, but they can capitalize on a feature that their propellers already possess: the ability to change blade pitch.
With a variable-pitch propeller, pilots can adjust the pitch of the propellers using a control in the cockpit. Altering the pitch affects how much the propellers bite into the air to reverse the direction of thrust. Many variable pitch propellers can adjust pitch past zero to a negative angle. With this setting, the propeller reverses direction, pushing air forward and slowing the aircraft down.
The Benefits of Reverse Thrust in Propeller-Driven Aircraft
Propeller-driven aircraft often operate on short runways or have steep approaches, like at St. Barts Island. Reverse thrust comes in handy in these situations.
Another advantage is for aircraft that land on water, such as floatplanes or flying boats, as they cannot use wheel brakes. Reverse thrust comes in handy in maneuvering and slowing down on water, allowing these aircraft to pull out of docks, make tight turns in water, or stop on short runways.
Reverse Thrust: The Pilot's Perspective
So, how do pilots engage reverse thrust? Here's the process:
- As the aircraft is on short final, the pilot moves the throttles all the way back to idle. The auto-brake system activates as soon as the main landing gear touches down and spins up to speed.
- At this point, the pilot deploys the reverse thrust system. Most aircraft equip separate control levers to operate the reverse thrust system. When the throttle is idle, the pilot pulls the reverse thrust levers backward. This opens the reverse thrust doors and spools the engines up as the pilot pulls them all the way back.
- Pilots typically keep the engines at a high reverse thrust setting for only a few seconds as the aircraft decelerates below approximately 80 knots. The pilot moves the reverse thrust controls to the idle reverse position when the aircraft decelerates to this point. The engines throttle back to idle at this setting, but the reverse thrusters remain deployed.
- The pilot eventually moves the reverse thrust levers back to the stowed position, which closes the doors again and returns the engines to normal forward idle. The rest of the taxi does not involve reverse thrust at all.
- The role of engines in aircraft landing goes beyond mere transportation; reverse thrust is a crucial technique used to slow the aircraft post-touchdown, reducing brake wear, shortening landing distances, and improving safety.
- Reverse thrust systems are designed with various methods to redirect exhaust gases or fan airflow, such as movable panels behind the engine or altering propeller blade pitch.
- In propeller-driven aircraft, reverse thrust can be achieved by adjusting the pitch of the propellers using a control in the cockpit, allowing the propeller to reverse direction and push air forward, slowing the aircraft down.
