Flight Level: Basics, perceptions and practices - II

Reyjo's Notion Series:

?Episode 1, Part II

Which flight level shall I choose while planning a flight?

As indicated in Part I, this is a discussion on conditions that influence the selection of flight level/ altitude for a short duration flight.

Let’s take an example of a general aviation airplane which intends to fly from Place A to Place B. The great circle distance between the city pairs is 100NM. This aircraft is a twin engine turbo-prop.

A rule of thumb that I came across is to put either the same three digits of the route distance (not great circle distance) or a couple of levels below or above this figure in your flight plan. This however works well in certain conditions but not for all.

Flight Rules: First you need to decide whether you are flying Visual Flight Rules (VFR) or Instrument Flight Rules (IFR). Just basics right! But it’s interesting to note that countries where the major airways are oriented north/south (e.g., New Zealand, Italy, Portugal) have semicircular rules that define a North/South rather than an East/West track split.

Terrain Clearance: Imagine that you are operating from airports that are at a higher elevation coupled with obstructions around or en-route. The application of the thumb-rule above will lead to errors, especially if you are unfamiliar with the geography of the operating area. On rare occasions, this error comes to the fore when the general aviation is supported by planning services that are operating off-shores. 

Departure and Arrival Procedures: Check for the transition phases of Departure Procedures (DPs) at Place A and Standard Terminal Arrival Procedures (STARs) at Place B to see if you need to be at a particular Flight Level or altitude for transitions that may be way above the thumb-rule. The design and extent of DPs and STARs varies widely across regions and there may be one or more existing conditions that restrict you to take a direct route at lower altitude.

Understanding of the airspace strata: The altitude separating the low and high airway structure varies from country to country. For example, in the United States it is 18,000ft whereas it is 19,500 feet in Switzerland and 25,000 feet in Egypt however, in certain countries this stratum is more regional and subject to change seasonally based on temperature/pressure variations. You will select the nomenclature altitude or flight level based on this.

Flight level and airspeed:  This may not be a criteria for selecting a flight level; however it is good to keep it in sync with the regulations depending on the nature of operations. 

Operational reasons: This is a point to consider most importantly for general aviation, owing to the wide spectrum of flying operations they undertake.

Equipment limitations: Inoperative aircraft system(s) will restrict your choice to opt for your preferred flight level.

Weather Conditions: Presence of known icing conditions, turbulence and convective weather are all factors affecting the selection of flight level. Adverse weather conditions most likely involve a longer route to circumnavigate such conditions; which increase the in-flight time. You need to pick a favorable flight level making use of these forecast conditions.

Tower En-route Control (TEC): TEC is an Air Traffic Control program in the U.S to provide a service to aircraft proceeding to and from metropolitan areas. The program’s intent is to provide an overflow resource in the low altitude system which would enhance ATC services. These have very specific and limited altitude options that you need to stick to; during planning phase. Such restrictions are present in bits and pieces across the globe in some way or the other.

Traffic Considerations: There are required routes published to enhance airspace capacity due to Temporary Flight Restrictions, both scheduled and unplanned, spread across different days/weeks of a year. There are also other forms of traffic advisories that are more dynamic in nature. First thing to be noticed in such situations is that the play books are designed for better Air Traffic Management and not to ensure fuel efficiency. In short, you should be on top of your game to ensure that you can fly what you planned by planning what you could fly!

Aircraft Performance: Airplanes are designed to operate effectively while serving a specific purpose. In a way, the flight level you pick should aim to complement its purpose.

Is it true that the ‘sweet spot’ in the atmosphere can only be leveraged by the privileged; or the more privileged don’t care much to fly within the crowded ‘sweet spot’?

Commercial airplanes are a fleet that put their ‘spotlight on sweet spots’. We will discuss more about sweet spots and drift downs in Part III; conditions that influence the selection of flight level for a commercial airplane in mid/long range flights.