flight dispatchers

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Today we will talk, in the form of homage, about the importance and what flight dispatchers do, a key position for air operations.

Few jobs are as thankless as being a flight dispatcher for a aircraft air cargo transport, nobody remembers them when the plane leaves like an angel from some critical airport (height, short track temperature, obstacles etc.), Nor do they remember them when the aircraft leaves so heavy or poorly loaded that the pilot feels that the blue planet is coming down on him and calls upon all his expertise and experience to obtain a successful, albeit precarious, climb.; at that moment it is not the dispatcher himself who comes to mind but the being that brought him into the world.

Post written by: William Casalins
Aeronautic engineer
FIA Foundation

flight dispatchers

Hardly anyone gives them the importance they have., ¡ni ellos mismos!. Many aircraft and lives have been lost due to poor clearance procedures, because the aircraft were heavier than reported, badly loaded (misplaced the load), the cargo was loose or they were poorly packaged dangerous goods etc..

The planes, whether ultralight or a jumbo jet, They are machines that must be perfectly balanced; the manufacturer gives exact instructions for weight and position of the center of gravity (C.G.) and the tolerances where this can go without hurting performance, the stability or structure of the aircraft.

The manufacturer, what's more, according to the characteristics of the airport of departure and destination (height, temperature, RH, wind, length, inclination and type of runway surface, etc.) gives maximum operating weights; It should be noted that these weights are calculated with the critical motor inoperative in V1, ósea el motor que más lo afecte´(when it fails) at the most critical moment, where the pilot must decide whether to take off or abort takeoff and brake, to stay within what is left of track available.

The first Creole flights were made according to the manual, after the pilot (who normally owned the plane or airline) when he got confidence he told the dispatcher (almost always empirical) "SALLY GATHERED, échale una toneladita más” and then one more and so on until the plane went really wrong, at the edge of the track and narrowly avoiding obstacles (with all engines operating at full power) Then he said "well, lower it half a ton and that's it", those were the track analyzes they used and they never took into account engine inoperative condition in V1.

In my judgement, a poorly loaded plane is more dangerous than a very heavy plane; And if those two events are combined ¡DIOS NOS GUARDE DE UNA PERDIDA DE MOTOR EN EL DESPEGUE! Some pilots have already been heard saying nervously “how can an ADF go away? (radio locator) we fall", the foregoing, alluding to the fact that if there were to be an engine stoppage there would be nothing to do, as indeed has happened in some twin-engines that are so difficult to control and even if they are correctly loaded, exceptional expertise is needed and more than that, what they really need is a place to fall, which unfortunately is not always available due to the large number of buildings near the slopes.

The trajectory of an object moving in the air is really that of its center of mass or center of gravity. (C.G.) and an aircraft is no exception, for his C.G.. pass the 3 airplane control shafts, which is achieved by airfoils that exert a force proportional to the speed and angle of said surface or stabilizer relative to the relative wind and distance (lever arm) from where this force is applied to the center of gravity (C.G).

Since the beginning of aviation there was a serious problem of stability and control, almost all the aerodynamic parts of the plane are destabilizing (the plane tends to worsen the situation as a function of time) the pilot had to maintain this control at all times with great effort and concentration; the pilot is human and could only perform this action for small intervals of time before ending up physically and mentally exhausted.

At first this was enough, but as the flights got longer and the planes bigger, it became a priority that the aircraft self-stabilize aerodynamically, this was achieved automatically only if we put the C.G. ahead of the center of pressure (C.P.), point where lift is considered to be concentrated) and depends on the area of ​​the wings, the queue and its distances, but that, as we will see later, has disastrous consequences for performance and weight, because as we say in engineering : nothing is free in life, And everything has to be paid for in one way or another.”, en aeronáutica se le dice “compromiso” y es mejorar unas cosas importantes sacrificando otras que no son prioridad para ese diseño en particular.

The main wing is the one that provides the greatest amount of lift under normal conditions., provides an upward force equal to the weight of the aircraft or sometimes much more (depending on the maneuver being performed). In a straight and level flight, as it is in almost the entire cruise (is the one that spends almost most of the flight time), the wing in addition to supporting the weight of the plane has an additional load, which is the downward force of the tail due to having the C.G. advanced (see graph).

It is obvious that being the C.G. the most advanced or the heaviest aircraft, this value tends to increase and to demand more upward force from the wings and this leads to more resistance to advance, more weight because the structure has to be made stronger and higher fuel consumption. On the contrary, if the weight were behind the aerodynamic center, the vector provided by the horizontal stabilizer would point upwards and while consuming additional power due to added drag on the tail (same as the previous case), this force is added to the lift created by the main wing, which means that it must work less and for that reason have a smaller structural resistance, lighter and have less drag, which translates into less fuel consumption and more speed, albeit to a substantial detriment to the stability and control of the aircraft. Afortunadamente en la actualidad cuando los diseñadores de aviones le han dejado a la electrónica la “responsabilidad” de la estabilidad, the operation of the control surfaces and the remotest possibility of stalling, aircraft with artificial stability are being built and given the advantages in performance and fuel economy are highly desirable; this really started in military aviation when fighters needed to be less stable, that in the end they were quicker in the maneuvers so that they could more easily evade enemy planes and missiles. With artificial stability systems, slightly unstable aircraft can be designed that would have better performance, that only powerful and redundant computers could keep them aloft.

The combination of having an over-heavy aircraft and with the C.G. too far ahead is highly undesirable especially in takeoff conditions, approach and landing, when the high lift surfaces give it a very strong nose down pitching moment, In addition, the low speeds of these maneuvers mean that the elevator and its horizontal stabilizer do not have enough authority. (force) to keep the nose up, and the loads imposed on the wings and nose landing gear could cause them to collapse.

As we said before, the position of the C.G. with respect to the aerodynamic center is what most affects the stability in the airplane; stable if ahead, neutral if they match and unstable if it is behind.

As a picture is worth a thousand words, we are going to use several graphs (exaggerated) to better understand all this mess. We will start with the takeoff or landing condition, with the C.G.. advanced, and on a cruise with C.G. late.

Warning: math ahead

Hahaha it's a joke.

static and dynamic stability

Static stability is the natural tendency of the airplane to try to return to its original attitude when it is disturbed in its straight and level flight by a gust of wind or any other reason..

Dynamic stability consists of the way in which the airplane tries to reach or arrives at its original flight state after said disturbance..

¿Cómo afecta la posición de C.G. stability and control?

The alas are the ones that fly and the total weight of the plane must be very close to the point where the sum of all the lift forces occurs (pressure center), otherwise the moment (the arm of the lever multiplied by the intensity of the force) between weight and lift would be so great that the elevator surfaces would be insufficient to maintain stable flight, especially at low speeds. For example, if there are conditions of heavy weight and too much lead added to the low takeoff and landing speeds together with the nose down pitching moment due to the deployed high lift surfaces, would be the most unfavorable event for control, performance and structure, but better for stability. The most likely solution is to resort to increasing speed to make the force of the tail more effective, use less degrees of flaps and thus decrease the positive pitching moment.

in performance (performance)

When the aircraft has a nose down pitch moment (positive), the tail has to do some downward force. This will depend on the size of the moment and the length between the horizontal stabilizer and the C.G.) We can consider this downward force as an additional weight that logically has to be supported by the main wings., that requires more drag, that is, more power available from the engines.

in the structure

Analyzing the worst event (nose-heavy and over-heavy at take-off), we said that the wing also has to take responsibility for the additional weight, This means that the stabilizer is pulling down to counteract the positive pitch moment. (nose down). This suggests that the wings have to be stronger and logically heavier., on landing the nose gear will be much more stressed (and everything in general) since the pilot does not have enough control to land smoothly on the runway; if it is the other way around (heavy tailed) it would be a miracle if the pilot put it in the right place on the track.

The aircraft dispatcher is a very important link in the aircraft operation chain.; as are the flight attendants who are not there to load the suitcases or serve the coffee, but they are really there to get us off the plane in less than 90 seconds in case of an emergency evacuation. The dispatcher is not there to load the plane, he is in charge of ensuring that the flight is prepared correctly, with the right fuel to go to the destination airport, to alternate and fly over the determined time before landing, make the flight plan for the most suitable route and height, depending on weight and winds, informing the pilot of the weather conditions at all airports and airways involved, aircraft status and maintenance notes, that dangerous goods are well packed, documented and their position well defined, people on board etc.. etc. HE, together with the pilot in command, They are the ones who, after all, plan the flight; that is why every time a plane makes a safe and on-time landing at a destination airport, it is because in large part the flight dispatcher HE DID HIS JOB WELL.

Some examples can be investigated:

• Overweight without consequences: Max Conrad, who set world distance records aboard a Piper Twin Comanche carrying twice the permitted payload by flying nearly 12,700 Km between South Africa and Florida.
• The accident of Boeing 747 in Afghanistan is an example of not exceeding the weight but the poorly secured load shifted backwards during takeoff.
• One that never fails in CRM courses is the ValuJet DC-9 crash that caught fire from mismanaged dangerous goods.

Thanks to my friends FIA, Jaime Sada and Freddy Higuera for their collaboration on this article.

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