Nowadays, commercial airplanes do not fly with metal wires moving the ailerons anymore. Now it is managed digitally, from the source (the sidestick or yoke movements) to the control surfaces movement. This system is called Fly-by-wire.

The fly-by-wire system consists of some computers (5 computers on board the Airbus’ planes, 3 primary and 2 secondary), that read the pilot movements, interpret them, and it sends a modified signal (from the original one) to the control surfaces. 

All these means that the airplane movements will not match proportionally what the pilot is doing actually. This philosophy shocked a lot of pilots, they couldn’t imagine flying something without a cable between the yoke and the surfaces. Finally, it was demonstrated that this system was much efficient, safer and much versatile.

Airbus was the first one using fully fly-by-wire (A320 were the first) and they create a set of laws that would govern this system. There are 4 laws: normal law, alternate law, direct law and mechanical back-up depending on the degradation of the airplane.

• Normal Law: It is on when normal operation is carried out. It controls the 3 axis (pitch, roll and yaw) balancing the aircraft and interpreting the sidestick movements. In addition, it offers some envelope  flight protections (on pitch attitude and roll attitude) and alleviate load factors, like the gravity when turning for a much better comfort.
• Alternate Law: If a minor failure exists (sensors disagreement, computer blackout, hydraulic system upset,etc) this law becomes the active one. Basically it provides the same control as the normal law, but without protections. Also, the roll control becomes “direct law-like” control.
• Direct Law: If a major failure exists (radio altimeter failure when approaching, 3 inertial references down, etc) this law become the active one. Then, the movements of the control surfaces are the same as the sidestick movements. There is no auto trim, and any protection system will be available (even alpha-floor for stall warning).

• Mechanical Back-up: This mechanical aid should be used when the electrical system is all down. Then, the airplane can only be controlled with the trim wheel for pitching, and rudder pedals for rolling.

The ACAP document is published by each aircraft manufacturer and it is used in airport design, not only for maneuvering area but also for terminal buildings. In addition, this document enables an optimization of all the airplane services around the gate.

In order to make a more pleasant explanation, we will use the Boeing 747-400 ACAP as an example. We will see the main parts of the the file, 6 important parts:

1.- AIRPLANE DESCRIPTION: Describes the dimensions of the aircraft, the exterior ones (size, gap between gear, the clearance between wing tips and ground, etc…) and the interior ones (load zone, cabin, cockpit, seat configuration, etc…). It also describes the operational weights like MTOW, MZFW or MLW for each model and engines set.

B747-400 door layout with all distances from the airplane’s head, in order to build proper jetways.

2.- PERFORMANCE: It’s made up with several sorts of charts, explaining the aircraft performances (that concerns airport designing). The two main charts are the payload vs. range chart and the take-off distance vs. weight. The first one, connects the maximum range as a function of the payload. The second chart, connects the aircraft wheight with the take off distance in different conditions. 

Take off distance vs. take off weight chart.

3.- GROUND MANEUVERING: It points out the parameters that are really necesary to move the airplane on ground. There, you can find ground-turns radius, cockpit visibility and different paths when steering.

Front gear path (red line) and main gear path (blue line) when 135º turn.

4.- TERMINAL SERVICING: It classifies the aircraft assistance on terminal (handling service). The ACAP gives you information about turn-around times for each service (fueling, cleaning, catering), ground services procedures and energy sources that is needed.

Sockets and control panels for ground servicing.

5.- JET ENGINE WAKE AND NOISE DATA: Explains the noise and jet wake factors with charts. The main usefulness is to avoid certain major accidents that may occur with these two sources. 

Jet engine speed diagram for take off thrust.

6.- PAVEMENT DATA: It details the forces exerted to the pavement. That determines what kind of pavement in use where an airplane can taxi all over (concrete, tarmac, etc…). Usually all these data is refered to gear pressure or gear tracks.

At the end of each ACAP, the aircraft manufaturer announces upcoming models, and aircraft draws as well.

The ACAP documents are typically public. Nevertheless, Airbus ACAPs are not public. But Boeing’s are. You can download them and take a look at Boeing Website.

Please, if you do not understand something or you just have questions, feel free to contact via comments. Don’t be afraid, it’s not that easy sometimes.

The ETOPS (Extended-range Twin-engine Operational Performance Standards) defines the twin-engine aircrafts requirements to operate flights where the nearest enroute alternative airfield is further than 60 minutes.

At the beginning of commercial aviation, because of the lack of regulations concerning alternative airfields further than 60 minutes, the air carriers started putting pressure on Aviation Administrations to be modified,  in order to be able to operate transatlantic routes with twin-engine. Because of that, ICAO and the FAA drawn up a new reguation that allowed to operate those routes. 

Nowadays, there are several ETOPS ratings depending on some parameters. These parameters are including the engines and systems’ reliability, crew training and ratings, manteinance and so on.

Author Andrés Meneses

These are the different ETOPS ratings issued these days by Aeronautical Authorities:

• ETOPS-75
• ETOPS-90
• ETOPS-120/138 (138 minuts is a 15% plus 120 minuts, in order to cover a little part of the Atlantic Ocean, not covered with ETOPS-120)
• ETOPS-180/207

An ETOPS rating is gradual. That means if you want to reach ETOPS-120 rating, first of all the aircraft must have reached the ETOPS-75 rating (with 200 sectors with 98% relieability), then the ETOPS-90 rating (with 300 sectors with 98% reliability), and finally the ETOPS-120. For example, in order to achieve ETOPS-120 rating, the airplane must prove less than 0.05 per mil in-flight shut-down. That means, flying 20.000 flights there’s only one in-flight shut-down (obviously, an airplane does not achieve 20.000 routes in its life).

There’s a tremendous application called Great Circle Mapper where you can compute each ETOPS rating maximum range around the earth.