Introduction to flight planning concepts. The use, selection criteria and definitions of Alternate Aerodromes

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One of the reasons why aviation nowadays is so safe nowadays is due to several significant factors, please find just a few mentioned below which left a significant mark on flight planning and monitoring in general and specifically catering for contingencies in the process:

1. Strict Regulations: Aviation is governed by rigorous safety regulations and standards imposed by national and international aviation authorities. These regulations cover aircraft design, maintenance, pilot training, air traffic control, and airport operations, ensuring a high level of safety across the industry.
2. Improved Training and Education: Pilots, maintenance personnel, and air traffic controllers undergo extensive and standardized training programs. This education equips them with the necessary skills and knowledge to handle various scenarios, emergencies, and situations, reducing the likelihood of accidents.
3. Safety Culture: The aviation industry has fostered a strong safety culture, emphasizing open communication, reporting, and learning from incidents or near-misses. This proactive approach to safety allows for continuous improvement and risk mitigation.
4. Sharing of Safety Information: Airlines and aviation organizations regularly share safety-related information, best practices, and lessons learned. This collaborative approach promotes safety across the industry.
5. Incident Investigation and Learning: Thorough investigation of accidents and incidents helps identify root causes and implement safety recommendations to prevent similar events in the future.
6. Risk Management: The aviation industry employs a systematic approach to identify and manage risks, helping to prevent potential hazards and improve safety.

Collectively, these factors have contributed to making aviation one of the safest modes of transportation today. While no system can be entirely risk-free, the continuous commitment to safety and improvement has significantly reduced the number of aviation accidents and incidents.

?In this light I wanted to share some basic knowledge about the different types of alternate aerodromes we generally have for commercial flight operations, as these are our insurance policy, as operators in case we cannot continue our flights as originally planned.

?Alternates aerodromes are selected based on numerous criteria:

1. Distance to the (projected) position along the route of the aircraft, depending on its phase of flight. The closer the better of course, but sometimes it is better to cater for a more distant but more adequate and weather permissible alternate if the forecasted weather conditions are marginal or other required criteria are less-than-ideal. (e.g., limited fuel availability, handling strikes, political instability)
2. Opening and eventual closing times and/or availability (due to Work in Progress) of the different required facilities.
3. Rescue and firefighting capability and availability related to the aircraft category we are operating and the status of the alternate for our respective flight.
4. The status of the alternates status according to the operating airline: Nearby alternates (planned in generally good weather) vs commercial alternates (planned in case a diversion is realistic based on the forecasts, as they have more facilities for handling, customs immigration etc.)
5. Weather. Especially cross or tailwind component, visibility, Cloud base.

The 2 official terms we use in this regard are:

1. Departure, Destination, and En-route aerodrome adequacy in terms of the applicable available facilities in such as:

• Air Traffic Services (ATS)
• Appropriate lighting
• Meteorological reports
• Navigational aids
• Emergency services
• Fuel and handling availability
• Appropriate apron pavement bearing strength.

The strength of the airport's pavement is given in a Pavement Classification Number (PCN), which includes details about the type of pavement (rigid or flexible), the quality of the ground underneath, tire pressure, and evaluation method. Aircraft manufacturers provide an Aircraft Classification Number (ACN) for each aircraft type, which considers the weight of the aircraft, the landing gear setup, and the range of tire pressure. By comparing the PCN and ACN, we can figure out the maximum weight an aircraft of a specific type can have for safe take-off and landing at that airport. If the PCN is higher than the ACN, the aircraft can be used without any restrictions on that pavement.

?2.?????Departure, Destination, and Alternate aerodrome weather permissibility in terms of the applicable weather forecast during the expected time window it will be used.

A "weather-permissible aerodrome" is an airport where the weather conditions are expected to be good enough for safe takeoff and landing. This is determined by checking weather reports or forecasts to make sure that visibility, cloud height, and wind meet or exceed the minimum requirements for operating at the airport.

The required minima are based on the aerodrome equipment on the ground and the aircraft's equipment in the air. These factors are especially important when comparing the forecasted or actual weather conditions during landing to the required minima.

A detailed explanation of what these required minima are will be given in another post as it is also a quite extensive subject. Not only does the Required navigation equipment (be it GPS, radio signal or any other means important but also runway lighting.

Additionally, the runway surface condition must be reported as safe for landing runway characteristics affecting performance (e.g., slope, length, obstacles), all within a predefined time window of expected use (usually +/- 1 hr.), but this time window can be larger for contingency airports along the route.

Types of Alternates:

We have a Takeoff Alternate, En- route Alternates, Fuel Alternates, Destination Alternates and ETOPS Alternates.

Takeoff Alternate:

A Take-off alternate is an aerodrome at which an aircraft would be able to land should this become necessary shortly after take-off and it is not possible to use the aerodrome of departure. This could be for various reasons, be it performance related, visibility/cloud base, unfavorable wind(gusts), congestion, thunderstorms or dust storms, volcanic activity, or any combination of the above.

En-route Alternate:

En-route alternate: An alternate aerodrome at which an aircraft would be able to land if a diversion becomes necessary while en-route (may also be the takeoff and/or destination aerodromes)

Fuel Alternate

A Fuel ERA aerodrome means an ERA aerodrome selected for the purpose of reducing contingency fuel. The fuel ERA aerodrome should be located within a circle having a radius equal to 20 % of the total flight plan distance, the center of which lies on the planned route at a distance from the destination aerodrome of 25 % of the total flight plan distance, or at least 20 % of the total flight plan distance plus 50 NM, whichever is greater.

Destination Alternate

A destination alternate is an aerodrome to which an aircraft may proceed should it become either impossible or inadvisable to land at the intended aerodrome. You may have a distinction between a nearby alternate, where only a fuel and go can be facilitated or a commercial alternate which can offer more handling. The latter is chosen in case the chance of an actual diversion becomes more realistic, therefore necessitation ore support if circumstances so dictate.

ETOPS Alternate:

A bit of historical context as this alternate type is the most recent and a bit more complicated than the rest:

Early combustion engines were highly unreliable, and it was common for a 4-engine piston aircraft to show up at the airport with only 3 of its engines working. As a result, the twin engine aircraft were required to fly beeline paths to remain in range of an adequate airport. Aircraft that had more than two engines were not restricted by this rule of flight paths. Because of this, many transatlantic flights were flown by Boeing and Airbus airliners since they can fly more direct routes. Flight plans had to accommodate these restrictions. The standard maximum diversion time is 180 min nowadays.

ETOPS used to stand for Extended Twin-Engine Operations, and now is Extended Operations. Originally, it was a certification that permitted twin engine aircraft to fly routes which may, at the time, be greater than 60 minutes flying time from the nearest airport that is suitable for an emergency landing. The other meaning of ETOPS is also the more informally known as: Engines Turn or Passengers Swim.

ETOPS are operations up to the operators approved diversion time (under ISA conditions (so called standard atmosphere conditions) in still air) at the approved one engine inoperative cruise speed. Both airplane and operator must be approved. Flight crew, flight dispatchers and maintenance personnel must be trained. Please see below illustration for a better general understanding of why ETOPS flights are planned more efficiently than NON-ETOPS.

Below is an example ( link source: Simbrief, which is a flight simulator flight planning application) of how a typical ETOPS flight plan route would look like, visually, from Vancouver to Amsterdam, with Edmonton, Goose Bay and Keflavik as ETOPS alternates.

Operator’s Approved Diversion Time

The Operator’s Approved Diversion Time is the maximum time authorized by the Competent Authority that the operator can operate a type of airplane at the approved one-engine inoperative cruise speed (under standard atmospheric conditions in still air) from an adequate aerodrome for the area of operation. Actual planned Diversion Times will be mentioned on the Operational Flight Plan. Escape routes are mentioned where applicable in the in the operator’s region-specific operations manual. Predetermined escape routes are sometimes necessary because ETOPS flights and ETOPS Segments of flights are not necessarily flown only over the ocean, where you can fly in a straight line to the diversion airport, but can take place over mountainous areas as well (like Greenland, see illustration below, but anywhere in the world where there are mountains and there is air traffic)?

So, briefly, an ETOPS alternate is an aerodrome along the route which is suitable and weather permissible at which an aircraft would be able to land after experiencing either:

1.?????An Engine Shutdown (Abbreviated 1X in this context)

2.?????Decompression of the cabin (Abbreviated DC)

3.?????A combination of the 2 (Abbreviated DX) or other abnormal or emergency condition while en-route in an ETOPS operation. (180 min diversion time instead of the normal 60 min)

What are the similarities and differences in dealing with these different contingencies?

The main similarity is that you will need to descend from your optimum, 2- engine cruise flight level to a lower level.

The differences:

In the case of an engine shutdown (1X)

Drift Down is a procedure used in multi-engine aircraft when one engine fails during climb or cruise and the plane cannot maintain its current altitude due to losing a large part of it's forward thrust and simultaneously compensating for asymmetric thrust as well, as a consequence thereof(which in practice results in more drag) When all engines are working normally, the best cruising altitude depends on the aircraft's weight and temperature. Usually, this cruising altitude is higher than the altitude it can maintain with only one engine (called the OEI (One Engine Inoperative) Service Ceiling).

If an engine fails above this OEI Service Ceiling, the plane needs to descend, and that's when the Drift Down procedure comes into play.

During Drift Down, the pilot sets the remaining engine (assuming we have 2) to maximum power and adjusts the rudder to compensate for any turning effect. The autothrottle/auto thrust system may be disconnected, but it depends on the aircraft type.

There are two strategies for Drift Down: obstacle clearance strategy and fixed speed strategy.

Obstacle clearance strategy: This strategy aims to keep the plane at cruising altitude for as long as possible with a gentle descent. This is especially important where a safe, high terrain clearance needs to be maintained at all times. The pilot sets maximum continuous thrust and, if needed, disconnects the autothrottle. The target speed is adjusted, and the plane maintains the best engine-out speed until it gradually descends to the Drift Down altitude. After that, the plane continues to cruise at the best speed and maximum continuous thrust.

Fixed speed strategy: This strategy involves starting the descent at a higher speed, which results in a lower cruising altitude. The pilot takes the same immediate actions of setting maximum continuous power/thrust, adjusting the rudder, and disconnecting the autothrottle (if needed). The descent starts sooner, and the plane maintains a fixed speed during the Drift Down profile.

The choice between these strategies depends on factors like obstacle clearance requirements and compliance with ETOPS regulations for certain routes. The operator should clearly communicate which strategy to follow in their Standard Operating Procedures (SOPs) or flight plan.

Considering the information above, especially for large, intercontinental aircraft usually the 1-X scenario is more critical at the beginning of the flight in case there is also high terrain in the vicinity, reason being that the aircraft is at it’s heaviest, still carrying a high fuel load. This would result in a much lower drift down altitude than if this would happen at the end of the flight.

In the case of a decompression (DC)

A rapid decompression of the cabin mandates an emergency descent which is a maneuver for descending as rapidly as possible to a lower altitude (potentially, to the ground for an emergency landing). The need for this maneuver may also result from an uncontrollable fire, or any other situation demanding an immediate and rapid descent but for the purpose of this article we will focus on the decompression.

The objective is to descend the aircraft as soon and as rapidly as possible, within the structural limitations of the aircraft.

This article considers some aspects of airmanship which are applicable to all aircraft and situations.

Crew and passenger Protection

At the first indication of a pressurization problem or symptoms of Hypoxia, the flight crew should immediately don oxygen masks. Without supplemental oxygen, the crew can be quickly incapacitated by lack of oxygen, and, at typical turbine powered aircraft cruising altitudes, the Time of Useful Consciousness can be less than one minute in the event of an explosive or rapid depressurization, which is why a speedy descent is mandatory.

Initiation of an emergency descent is done as a memory item drill in most aircraft types.

Once the descent has been initiated, it is standard procedure to confirm that all required actions have been completed by referring to the appropriate checklist in the Quick Reference Handbook (QRH). The autopilot of many current generation aircraft can be used by the Pilot Flying to carry out an emergency descent profile and many manufacturers recommend that the autopilot be left engaged for the maneuver.

Unless structural damage is suspected, the aircraft should be descended at or near maximum speed (Vmo) with thrust at idle and flight spoilers or speed brakes extended. If structural damage is suspected, the aircraft should be flown at, or close to, the indicated airspeed (IAS) at which the failure occurred.

Contrary to the 1X scenario, it’s normally speaking the DC scenario which is more critical at the end of the flight in case there is also high terrain in the vicinity, reason being that the aircraft, irrespective of its weight would need to descend to 10.000 feet to ensure enough oxygen available in the air for the passengers, where fuel consumption is considerably higher than at it's optimum high cruising altitude, in a situation where you already don’t have a lot of fuel remaining in tanks due to the fact that most of it has been used already..

In the case of an Engine failure and airplane depressurization (DX)

Emergency descent at Maximum speed, and speed brakes extended) down to FL100 or applicable Minimum, as in the DC scenario.

Flight Altitude (MFA) whichever is higher.

Diversion cruise at the speed schedule or with LRC one engine out speed, depending on airplane type, as in the 1X scenario.

The procedures established by the operator should ensure that ETOPS is only planned on routes where the Operator’s Approved Diversion Time to an ETOPS ERA can be met, which in practice could mean that next to satisfying the “regular” diversion time requirements( “normally “diverting in a straight line) you will need to ensure that your diversion aerodrome(s) in mountainous areas can at any time be reached when necessary, in each of the 3 aforementioned scenario’s (1X, DC,DX), while flying the escape routes, or catering enough fuel as a certain percentage to ensure that the alternate can be reached safely.

Note: The Approved Diversion Time is only used for determining the area of operation, and therefore, is not an operational time limitation for conducting a diversion. Prevailing weather conditions, availability of ETOPS ERA’s or other factors such as technical limitations can influence actual diversion time.

There are still a lot of considerations and operator specific procedures which could be explained however the scope if this article is to explain the general, non-operator specific concept of considerations when choosing an alternate.

Note: I would?like to thank my co-writer Terry Mitchell for his contribution to the design of the article and for ensuring readability also for people outside the aviation industry