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In the red corner – weighing a Boeing 787

By: Dave Gunner

February 19, 2021

A few blocks of wood, a tape measure, a clipboard and a pen. These unlikely items are used to take one of the most critical measurements of one of the most sophisticated jets in the sky. We're talking about weighing an aircraft, why that's important and why it must be done regularly.

The author Douglas Adams once famously said that there was a simple knack to flying – all you have to do is throw yourself at the ground and miss. If only it was that easy. When an aircraft lines up on the runway ready for takeoff, many complex calculations have been made to determine its performance and the speed it need to be going to lift off the ground. Many of those calculations are based on weight and are built up from the empty aircraft’s dry weight. It’s important that we know exactly what that is, the trouble is, it keeps changing!

A short video showing the weighing of G-VZIG

Over time there are hundreds of different things that can change the weight of an aircraft including modifications, repairs, new carpets and different seats. These are all strictly monitored and logged for each aircraft by our flight operations engineering, and each aircraft is different. Although the team has a good idea of these weights, just to be on the safe side, we bring each aircraft into the hangar and pop it on the scales. We do this every couple of years.

Emptying the tanks

Transferring fuel from one aircraft to another. For this particular job our engineers pumped G-VZIG’s fuel into two different aircraft which is why the eagle eyed viewer will spot an A330 and a B787 in the video.

Weighing can only be done inside the hangar (we’ll get to that in a second) but before the weighing starts, the fuel tanks need to be emptied. That’s done by transferring the fuel to another aircraft parked behind it. This process can take a few hours depending on the aircraft types and how much fuel is still on board. There is always a small residual amount of fuel remaining which is then removed through a series of valves under the wing.

Preparing the cabin

The next task is to go through the aircraft with a fine-tooth comb, removing everything we added after delivery. This includes inflight magazines, pillows, blankets, and safety cards to name but a few. Even coat hangers and toilet rolls come off. Simultaneously the engineers complete a basic equipment checklist recording what’s included in the basic weight. This list is double-checked against the aircraft technical records.

Once the aircraft is prepared it’s time to close the giant hangar doors. This is vital because even the slightest breeze can affect the reading on the scales. Rain and temperature can also skew the results, so it must be done in the hangars closed, controlled environment.

Stepping on the scales

Now to the weighing itself. We work with a company called PlaneWeighs who turn up in a van loaded with everything they need. That’s to say the scales, some bits of wood and a tape measure. Carl from PlaneWeighs builds the scales in front of each wheelset, one for each wheel so ten for the Boeing 787. The scales are flat plates 5cm tall, just big enough for the tyre to sit comfortably.

Once the scales are in place and everyone is happy, certifying engineer Mike Morland is positioned on the flight deck to operate the aircraft parking brakes. Afterwards, Mike will be weighed to be subtracted from the aircraft’s overall weight (he was a trim 90kg!)

G-VZIG on the scales

The aircraft is then unplugged from all the ground supplies and Mike releases the brakes. A tug, expertly driven by Arek Cieslewicz, pulls the aircraft forwards onto the scales.  Once the aircraft is correctly positioned on all the scales the tug is disconnected and the weight measurements from all ten scales recorded. This process is repeated three times to make sure the weights are reliable.

The final job is to check the aircraft’s centre of gravity. On the Boeing 787, the engineers use a plumb line that registers on a small grid attached to the central landing gear bay.

The final job is to check the aircraft’s centre of gravity. On the Boeing 787, the engineers use a plumb line that registers on a small grid attached to the central landing gear bay.

The verified weights and centre of gravity are given to Jason Read, Manager, flight operations engineering and efficiency, for a final reckoning. “My team tracks the aircraft weight and centre of gravity, monitoring all modifications that have a weight and balance impact. Therefore, we have a good idea what the resulting aircraft weight and centre of gravity will be, and that is what we approve,” said Jason. “We take the weight from each scale and any adjustments from the report and perform our own calculations to make sure we get the same final aircraft weight and centre of gravity as the report. Once we’re happy, the necessary documents are produced. New final weight and centre of gravity gets added to our flight planning and weight and balance systems and used to calculate the fuel requirements and trim calculations for that aircraft’s flights”.

The pilot’s point of view

Chris drops by the hangar to talk about weights and what they mean to a pilot

The next time that weight will be used will be in the build-up to a live flight. We asked Captain Chris Hall, a 787 pilot, to explain the different weight calculations needed to fly the 787 and why they’re essential.

“The most important thing for pilots in the setup of the aeroplane before we take off is to completely understand the weight of the aeroplane and most critically, the takeoff performance calculation,” said Chris. “That’s arrived at using several important weights that are all built from the aircraft’s basic dry weight. First, we add the catering’s weight, the potable water, all the magazines, safety cards and other non-standard items, which becomes the dry operating weight. We then add the weight of all the passengers, their baggage and any cargo we’re carrying. That gives us what’s called the zero fuel weight. We then add the fuel required for the mission, which for a long flight from London to Hong Kong can be around another 80 tonnes. That, minus what you burn on the taxi out to the runway, gives you your takeoff weight (the maximum takeoff weight for a Boeing 787-9 is 252,650 kgs). Once you know that, you can factor in local airport conditions such as weather and runway altitude and length to determine the different speeds needed for a safe takeoff. From this also comes the other important weight, which is the landing weight.”

Each main landing gear wheel carries a load of about 15 tonnes

Weighing aircraft is one of those unseen and little-known processes that make air travel safe. And it’s one small example of why we think aviation is the ultimate team game. Those weights, taken on this day in the hangar, verified by our operations and engineering teams and used by our turnaround officers, along with weights from our airport and cargo teams, will be passed to pilots ahead of each flight so they can accurately calculate the speed they need to be travelling when they leave the runway. And in case you were wondering, the final dry weight of G-VZIG – Dream Jeannie –  is 123,303.6 kgs and the centre of gravity was within 0.06 inches of where expected.


With huge thanks to our amazing Heathrow hangar teams. Day 1 – Jason Wright – aircraft maintenance supervisor, Pete gilbert – maintenance assistant and tug driver, Cameron Junor-Graham – technician and Gary Oram – certifying engineer. On day 2 – Tom Minto – aircraft maintenance manager, Mike Morland – certifying engineer (manning the brakes), and tug driver Arek Cieslewicz.

 

Dave Gunner

Dave Gunner

I love telling the story of our people, our planes, our places and our planet through Ruby Blog.

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