Each conventional aircraft, just because they have wings, they create wake turbulences at wing tips. Since we have high pressure underneath the wing and low pressure above it, this pressure difference converge at the wing tips. Air from high pressure goes up to low pressure zones, plus the forward movement of the airplane create an espiral-like movement of rough air behind the airplane. These wake turbulences are increased (abruptness and size) when dealing with heavy airplanes and low speeds, like the approach stage.

Wake turbulence created by the airplane (NASA)

This rough air does not concern this aircraft, but the aircraft behind it does. Actually, the aircraft behind might fly through this turbulence. As the air is in rotation, it does not keep sticked to the wing foil, so it may cause a lift loss. That’s why ICAO establishes a minimum separation between aircrafts, enroute as well as in approach, in order to avoid rough air. These turbulences drive away because of air viscosity and because of the wind. Even that, they could be 5 miles long and go down up to 900 ft.

In order to decrease these effects, there is a very used and useful tool called winglet, set up at the wing tip, and it slows down the air flow reducing then, the wake turbulence (and the fuel consumption).

Winglet                        Source: Air Guide Online

Every single day, hundreds of commercial airplanes cross the north atlantic, flying transcontinental routes linking North America and Europe basically, the NAT (North Atlantic Tracks). The performances of these flights are much different and complex due to its distant routes from any kind of airport.

The twin-engine aircrafts operating these flights (usually big ones) must have the ETOPS rating, explained in other post. That is because the nearest alternative airport when flying above the atlantic is 180 minutes far. If an engine failure happens (or any other system), this aircraft should divert to the closest airport immediately.

Another huge problem is the radar coverage. As a matter of fact, this radar coverage does not exist. Radars must be set up on ground or close (not floating over the sea). The main Air Traffic Control facilities (Shanwick Oceanic for European side and Gander Oceanic for US side) are equipped with air traffic management systems that by means of pilot manual position reports, they have some sort of “radar-like” screen with all the airplanes’ positions.

The third and big problem as well (but solved anyway) is about communications. The communicacions between pilots and controllers use VHF (Very High Frequency from 30 MHz to 300MHz) frequency range. VHF waves only reach “line-of-sight” spaces. So, in that case they must use HF (High Frequency from 3 MHz to 30MHz) that bounces off the ionosphere and give coverage to greater distances. Nevertheless, even this advantage, sound quality is much poor.

The NAT routes are designed and published daily. They are defined with an entry waypoint, an exit waypoint and between, waypoints are defined with coordinates (there are no navaids to define them). Early in the morning, westbound routes are published. Then late at night, eastbound routes are published. Europe incoming routes are usually defined at higher latitude, to take advantage of the Jet Stream (high speed wind, will post about this).
By the way, pilots flying these routes cannot eat the same meal.

Here we can appreciate North Atlantic Routes all the day long

The quick fuel draining system (called Fuel Jettison) is an onboard system in large aircrafts like B747, B767, B777 and some B757 and also A330 and A340.

When an emergency procedure around an airport is carried out, the airplane must be able to land with the Take Off Weight (sometimes Maximum Take Off Weight). Then, the low/mid range airplanes have no problems dealing with that, but long range airplanes does.  

Heavy airplanes usually carry a huge amount of fuel (because of long haul flights). Thence, according to JAR regulation JAR 25.1001 about Airworthiness, these heavy aircrafts must be equipped with a Fuel Jettison system. The regulation require to the system to empty the tanks in 15 minuts, and then leaving enough fuel to keep a 3.2% climb gradient in landing configuration at 1.15 stall speed. For example, a B727 releases 1060 kg (2330 lbs) of fuel per minute, with all tank pumps running. The fuel nozzle is set at the wing tip, in order not to make any damage at the airplane fuselage. 

    Source: Wikimedia

Even though this procedure seems pretty enviromentally harmful, it isn’t much critisized because its low frequency, low amount of sprayed fuel and the procedure conditions. If possible, fuel jettison procedure must be done over the sea.

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.