The Difference Between Heavy and Large Aircraft
Airplanes are divided into different categories, including large and heavy, based on their ability to take off at certain weights. For pilots, the weight-related categories are important because they need certain ratings to be able to fly bigger aircraft. For both air traffic controllers and pilots, the categories are crucial in notifying pilots of dangerous conditions in the wakes of the heavy planes.
Maximum Takeoff Weights
The Federal Aviation Administration defines a large aircraft as one that has a maximum certificated takeoff weight (MTOW) that's more than 41,000 pounds and less than 300,000 pounds. That means the plane is capable of taking off at a weight within that range; it would still be classified as large even if its weight at takeoff was 41,000 pounds or fewer.
A heavy aircraft has an MTOW of 300,000 pounds or more. A medium aircraft has an MTOW that's 12,501 to 41,000 pounds. A small aircraft is one with an MTOW of 12,500 pounds or less.
A pilot must have a specific rating to be able to legally fly large (or turbo-powered) airplanes. This rating can be acquired through a practical test, often called a checkride, on a highly realistic simulator or an actual airplane. The checkride is preceded by weeks of instruction and training.
Because the FAA recognizes two types of very big heavy aircraft as being extra hazardous to fly behind, they're described as super. These are the Ukraine-built Antonov An-225, which has an MTOW of 1.41 million pounds, and the Airbus A380-800 (also called the A388), which has an MTOW of 1.27 million pounds.
Boeing 757s Also Treated Differently
Boeing 757s are technically large airplanes based on their MTOW. But because they create air turbulence in their wakes more like a heavy aircraft, the FAA requires air traffic controllers to treat them differently as well.
Wake Vortices and Their Effects
The wings of planes of all sizes create vortices—counter-rotating cylindrical air masses—in the aircrafts' wakes whenever the wings are providing lift to the planes, including on takeoffs and landings. The vortices created by small, medium, and large planes (except for 757s) don't present special danger to other planes, but those created by heavy and super aircraft can produce turbulence that lasts a considerable time after the aircraft have moved on and can cause smaller planes to crash. For that reason, air traffic controllers say the word "super" or "heavy" when identifying nearby aircraft that fall under those categories, so pilots of other planes will know to stay clear.
In addition to the weight of the plane, the powerfulness of the vortices is affected by the speed of the plane and the shape and span of its wings. According to the FAA, a vortex speed of almost 300 feet per second has been recorded.
Vortices act differently at different elevations. They can be blown about by winds, and they generally sink and lose strength over time, though they may also rise under certain conditions.
When a smaller plane is caught in a vortex, it will be rolled at a rate the pilot may not be able to counter, causing the plane to crash. Even super or heavy planes can be adversely affected by vortices from a super or heavy aircraft.
The FAA requires air traffic controllers to separate planes traveling behind super and heavy aircraft by the following distances:
|Trailing Plane||Leading Plane||Distance|
|Small or large||Heavy||5 miles|