We all know countless celebrities and famous people who have adopted a stage name suggesting a curvy body. Just as in the world of celebrities, in aviation, heavy airplanes get the honorific tag line ‘heavy’ after their call signs. With that said, adding the word ‘heavy’ entails more work for the air traffic controllers and pilots. Roll up your sleeves and let us begin the story of ‘heavy’.
Federal Aviation Administration (FAA) classifies aircraft depending on their Maximum Take-Off Weight (MTOW):
Note: In the ICAO classification, medium and large categories are consolidated into one category ranging from 12,500 pounds (7,000 kg) to 300,000 pounds (136,000 kg).
The current weight of an aircraft does not change the position in the above classification; heavy classified aircraft remains to be heavy regardless of the instantaneous take-off weight.
Apart from the four weight categories, the super aircraft category is dedicated to two aircraft: the A380 superjumbo and the one and only AN225 Ruslan. And now, A380 is the only aircraft representing the super aircraft category with the demise of the AN225. These behemoths are aircraft with a maximum take-off weight exceeding 1 million pounds.
The prominent reason behind categorizing aircraft on their maximum takeoff weights is to mitigate wake turbulence issues. What does that mean?
Every object that flies generates a lift. Taking an airplane as an example, it generates the lift by its wings leaving a low-pressure area above the wing surface and a high-pressure area below the wing surface. At the wing tips, high-pressure air at the bottom leaks into the low-pressure upper area creating a wake. These counter-rotating cylindrical-shaped wingtip vortices are well-known for imposing rolling moments on the following aircraft; especially on smaller aircraft types. As the lift differs – an aircraft with a higher weight requires more lift – with the aircraft’s maximum take-off weight, strong wake turbulence is expected from heavy airplanes.
Boeing aircraft like 747, 777 and 787, and Airbus birds like A300; A330; and A350 are classified into the heavy aircraft category with a maximum take-off weight exceeding 300,000 pounds. Hence, these large aircraft leave strong wake turbulence.
The categorization is done based on the maximum take-off weights even though the real deal is the amount of wake turbulence generated by an aircraft. Hence, there are some exceptions to the 300,000 pounds rule, as there are airplanes with a weight less than 300,000 pounds but generate significant wake turbulence. In such cases, the relevant aircraft is put into the heavy aircraft category. Boeing 757 is a famous example with a maximum take-off weight just below 300,000 pounds margin and yet considered a heavy aircraft.
If you have ever got a chance to hear radio transmissions between the pilots and air traffic control, you would have noticed that air traffic controllers use the term ‘heavy’ with the call sign of certain aircraft.
Shamrock 934 heavy, cleared to land on the runway 24R.
Here shamrock is the call sign of Aer Lingus and tagged with the term heavy as flight 934 is a heavy commercial airliner. As you might have noticed in airline radio chatter, no one is using the airplane tail number but the flight number accompanied by a catchy call sign. British Airways owns the most catchy call sign – speed bird. Likewise, all commercial airlines have got their names for ease of identification.
A flight plan is issued to a certain flight number, and airlines can change the aircraft serving the relevant route without changing the entire flight plan. Hence, pilots and controllers stick to the call signs and flight numbers at all times. When the call sign and the flight number are accompanied by the term heavy during the communications, other pilots of nearby aircraft and controllers are alerted to leave additional separation.
Air Traffic Control (ATC) is responsible for maintaining a minimum distance between airplanes. The minimum separation distances differ from one aircraft category to the other. Hence, the air traffic controller should pose a patent understanding of the aircraft sizes as every aircraft under his supervision appears as only a blip on the radar screen.
To ensure safe operations, FAA requires airplanes flying under IFR, VFR (flying within class B, C, or Terminal Radar Service Areas), or radar sequenced VFR flights to maintain a minimum separation distance (in nautical miles) as follows:
1) Aircraft operating directly behind a super aircraft or heavy aircraft at the same altitude or less than 1,000 feet below,
2) A small aircraft operating directly behind the same altitude or less than 500 feet below a Boeing 757,
3) Small aircraft are separated near a runway from the time till the preceding aircraft liftoff.
Above restrictions are imposed upon the following aircraft if it is smaller than the preceding one. When the following aircraft is larger than the leading one, the minimum distance reduces, and only a separation of two and half nautical miles is required for safe operation.
Air traffic controllers generally apply wake turbulence separations for all flights flying under Instrument Flight Rules (IFR). If required, pilots can accept clearance to visually follow an aircraft without the help of ATC. Pilots flying under Visual Flight Rules (VFR) will receive a Wake Turbulence Cautionary Advisory when the separation drops below the threshold.
Wake turbulence generated by an aircraft differs depending on many factors. Wide-body aircraft with a higher weight and a broader wingspan generate strong vortices, especially when traveling at slow speeds. Heavy-slow-clean airplanes give rise to the most annoying vortices. Within the terminal areas, all the airplanes are flying slowly generating strong wake turbulence. Hence, during take-off and landing, controllers shoulder the responsibility of maintaining adequate distance between the planes to avoid wake turbulence disturbing other planes in the vicinity.
When taking off behind a large aircraft, applicable procedures differ depending on the runway configuration and the same is true for landings as well. Required distances and procedures for each situation are stipulated in the FAA Aeronautical Information Manual (AIM) under chapter 7 section 3.
Another pivotal fact to note is that wake turbulence tends to sink with time. According to the FAA, they sink at a rate of 300 to 500 feet per minute for up to 30 seconds. This is the reason why aircraft are not allowed to fly directly behind or 1,000 feet below the leading aircraft’s path.
If a smaller plane happens to fly through the wake turbulence of a large aircraft- especially at low altitudes- the results may be catastrophic as pilots do not have enough time to recover. The wake turbulence can induce rolling moments on a small plane when it passes through a rotating vortex resulting in losing roll control authority.
As depicted by the below image, an aircraft with a shorter wingspan where the ailerons fall within the rotational flow field will have the highest difficulty to counter control the rotation.
Even at high altitudes, the wake turbulence of a heavy aircraft can impose dangers on the other planes flying directly behind. Higher operating speed and air density are more prominent when calculating the minimum separation distances between the airplanes. The strength of the vortex reduces at high altitudes due to low air density, but the rotation speed increases making the strength equal to a wake at sea level. Airplanes fly much faster during cruising and it takes less time for an aircraft to penetrate the wake turbulence.
Pilots and controllers add the term ‘heavy’ to their call sign with the sole intention of reminding the size of their equipment to pilots in other aircraft and the controllers. The theory behind the call-out is to keep other aircraft safe from the wake turbulence. To make things more effective, FAA has mandated minimum separations in nautical miles for each aircraft category. Smaller airplanes are more susceptible to wake turbulence and should be operated cautiously when operating behind another aircraft.
Longing to discover more about the behavior of wake turbulence and related facts, here is a great source: Wake Turbulence Training Aid.
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