ICAO Annex 6 — Page 8, Lesson 13

I want to walk you through the opening of ICAO Annex 6 as it relates to operational procedures. We're starting with a key concept: how international aviation works between countries that have signed the ICAO convention. Let me explain that first paragraph. ICAO stands for the International Civil Aviation Organization, and a "contracting state" is any country that has agreed to be bound by the ICAO treaty. The key document here is Annex 6, which deals with the operation of aircraft. The passage gives you a practical example: suppose an airline registered in the United States wants to operate a flight into London Heathrow. The UK is a contracting state of ICAO, which means it complies with what are called SARPs — Standards and Recommended Practices. That compliance is sufficient evidence for the US operator to plan to use Heathrow, either as the destination aerodrome or as a nominated alternate. In other words, because both countries follow the same ICAO standards, the US airline doesn't need separate bilateral verification for every single airport they want to use in the UK. Now let's move to the core of this excerpt: alternate aerodromes. An alternate aerodrome is formally defined as an aerodrome to which an aircraft may proceed when it becomes either impossible or inadvisable to proceed to, or to land at, the aerodrome of intended landing. So it's your backup plan — a place you can go if your primary destination becomes unusable for any reason. The excerpt breaks alternates down into four specific types. Let me go through each one carefully. First is the take-off alternate. This is an aerodrome at which an aircraft can land should this become necessary shortly after take-off, and conditions are such that it is not possible to use the aerodrome of departure. So imagine you've just taken off and something goes wrong — an engine problem, a system failure — and you can't return to the airport you just left. The take-off alternate is the airport you planned to use in that scenario. The choice of take-off alternate is made during the planning of the flight on the day — it's not a fixed choice; it depends on the specific conditions of that day's operation. Second is the en route alternate. This is an aerodrome at which an aircraft would be able to land after experiencing an abnormal or emergency condition whilst en route — that is, while you're in the middle of your flight, not at the beginning or end. The choice of en route alternate aerodromes depends upon two factors: the type of aeroplane you're flying, and the areas over which the operation is taking place. Under normal planning, it is standard practice to select what are called 'operationally acceptable' aerodromes for this purpose. However, the excerpt makes an important distinction: in an emergency, any 'suitable' aerodrome could be used. So there's a difference between what you plan for — operationally acceptable — and what you can actually use in a real emergency — any suitable field. Third is the ETOPS en route alternate. ETOPS stands for Extended Twin-engine OPerationS. This is a specific type of en route alternate for twin-engine aircraft operating on routes where diversion airports are far apart. The definition says this is a suitable and appropriate aerodrome at which an aeroplane would be able to land after experiencing an engine shut down or other abnormal or emergency condition whilst en route in an ETOPS operation. The critical requirement here is that an ETOPS aeroplane must always be flying within 'range' of a suitable alternate aerodrome. That range is a calculated distance based on the aircraft's performance with one engine failed — you can never be farther from a usable airport than that distance allows. Fourth, the excerpt begins to introduce the destination alternate, though it cuts off mid-word. A destination alternate is an aerodrome to which you can divert if, upon arrival at your intended destination, you cannot land there — perhaps due to weather, a closed runway, or some other issue. We'll cover that in more detail when we continue. Let me show you a diagram that illustrates how these alternates relate to a typical flight route. That figure shows the destination alternate concept visually. You can see how the take-off alternate sits near the departure point, the en route alternates are spaced along the route, and the destination alternate is positioned near the destination airport. So to summarise what we've covered: an alternate aerodrome is your backup landing site when the intended landing aerodrome becomes impossible or inadvisable to use. There are four types — take-off alternate, en route alternate, ETOPS en route alternate, and destination alternate — each with its own specific definition and planning criteria. The take-off alternate is chosen on the day of flight for problems right after departure. The en route alternate depends on aircraft type and operating area, and while you plan for operationally acceptable airports, any suitable airport can be used in an emergency. The ETOPS en route alternate has the additional constraint that the aircraft must always remain within a calculated range of a suitable alternate.