As you may have noticed, I am not an aerospace engineer. In fact, I’m not an engineer of any discipline. Though over the last several years, I’ve sought to learn more about aircraft performance, payload capability and design to better understand the industry I cover and I was keen to share what I had learned. Many of you are aircraft designers, so your job is to check my work and recommend where this can be clearer. For those of you are like me – learning – I hope this will become an accessible reference. So, today I give you what may be the first installment of what may become a periodic series: Aircraft Design 101 (as told by a non-engineer). Physics for Poets, if you will.
Every aircraft has a Maximum Take-off Weight (MTOW), which is dictated by the structural capacity of the aircraft. Though within the aircraft’s maximum allowed weight are several different elements, each one contributing to the overall performance of the aircraft. If we think of MTOW as a glass which cannot be overfilled, inside sits layers of an alphabet soup of additional weights that determine how much an aircraft can carry and how far it can be carried.
Let’s take a large jetliner for example and load it to its maximum takeoff weight the moment it begins its takeoff roll. At this particular moment the total weight, or gross weight, of the aircraft is the sum of the aircraft, the amount of fuel it’s carrying and what it’s carrying. This is also known as the Take-Off Weight (TOW) That is of course an over simplification, but these are the three key ingredients to understanding how much an aircraft weights.
Before any items are installed that make the aircraft usable as a commercial transport, the aircraft itself its made up of the airframe, furnishings, the systems and its propulsion. The sum of these three items are the Manufacturers Empty Weight (MEW). The MEW also includes, for example, hydraulic fluid, which is found in a “closed” system aboard the aircraft and not consumed.
As it readies for revenue service, many items are added to the aircraft for it to be missionized. For example, the seats, emergency equipment and other consumable fluids such as engine oil, toilet chemicals and fluids, as well as the fuel that can’t reach the pickups in the tank, also known as unusable fuel. Naturally, you’re not going anywhere without the flight and cabin crew and their baggage. All together, you add the weight of these items to MEW to get the Operational Empty Weight (OEW or OWE) of the aircraft.
While it’s parked at the gate and you’re watching your ride from the terminal windows, the aircraft is loaded with fresh catering and potable water, your pre-flight newspaper, any pantry equipment and extra crew. Add these weights to the OWE and you have the aircraft’s Dry Operating Weight (DOW).
Once everyone is boarded comfortably ready to fly along with their baggage in the overhead bins and in the cargo hold, that weight is added with the pallets with revenue cargo that are flying along with you to your destination. All these items are called the Traffic Load (TL). The TL is then added to the DOW to yield the Zero Fuel Weight (ZFW), before an ounce of usable fuel has been put into the tanks. Every item into the ZFW that leaves the gate will arrive with you at your destination.
For the sake of this scenario, with everything loaded on board, now comes the the Jet A. The weight of all that gas you’ll need for your trip is made up of three elements. The first is your reserve fuel, enough for 30 minutes of cruise during the day or 45 minutes at night, which when added to your ZFW will give you your Landing Weight (LW).
The distance you’ll fly and the fuel you’ll need to fly there directly is called your Trip Fuel and is the largest portion of the fuel weight. As noted early, when the aircraft begins its take-off roll it is at its Take-off Weight (TOW), but when it pushes back from the gate it may exceed the MTOW, because the Maximum Design Taxi Weight (MTW) is higher than the MTOW. That’s because your pilots have requested extra Taxi Fuel that will be burned while moving the aircraft from the gate to the take-off position.
Returning to the idea of the aircraft as a vessel that cannot be overfilled by the sum of the different elements. In a perfectly efficient use of the aircraft, the aircraft would depart the gate at MTW, leave the runway at its MTOW, and land at its LW just before using any reserve fuel. In an operational setting, Trip Fuel and Traffic Load are the two variable elements. On a short flight, less Trip Fuel is required, allowing the carrying of additional revenue cargo for example, along with the full load of passengers and their baggage.
This post was originally published to the internet between 2007 and 2012. Links, images, and embedded media from that era may no longer function as intended.
This post originally appeared at Flightglobal.com from 2007 to 2012.
