Tag Archives: weight and balance

Refuelling or not refuelling?

Last week, I took my brother and sister onboard of one of my flight lessons. Ahead of the flight I reminded my instructor that I would bring them along and asked what flight route should I prepare. His response: “prepare the weight and balance report, the destination will be a surprise”. And so I did.

For those not initiated, each aircraft has a defined maximum take off weight (MTOW). Before the flight, the pilot needs to ensure that the aircraft will be below that weight. On top of that, the centre of gravity must be within certain limits. That is the weight and balance (1).

For a small simple aircraft like the Robin DR-44 we flew, it is a rather easy calculation that can be done with a pencil. See in the image below two ways of calculating it: making the numbers or using the graphic at the bottom of the image.

Weight and balance report for Robin DR-44.

Let’s review the numbers. We flew the DR-44 with immatriculation F-GSRR, which empty weight is 616kg. I estimated that the instructor and I, fully dressed and with headsets would weight ~160kg. My brother and sister behind, another 160kg. Baggages: I almost emptied mine and weighted it, 2.5kg. My instructor’s one is rather heavy, I assumed that together they would be 10kg. Principal fuel deposit: 110L of Avgas, with a density of 0.72kg/L, 79.2kg. Another 50L for the reserve deposit, 36kg.

Summing up: 1,061kg.

You can read in the image that the maximum take off weight for the plane is 1,000kg…

What to do then? Clearly, the aircraft is a given, so weight shall be reduced from somewhere else. But, from where? Either we left someone on ground or reduced baggage weight (my instructor left his and brought along only a book with aerodrome charts). However, baggage weight contributed only 10kg to the initial calculation. I then calculated: what is the maximum fuel we can carry?

Forget the reserve deposit: 36kg less. Let’s go with the principal deposit. What is the maximum fuel volume that would enable us to be within the 1,000kg limit? It would be somewhere about 80L (vs. the capacity of 110L of the deposit).

When I arrived to the aerodrome, I came with the message to my instructor: “Thierry, we can only carry 80L, if the plane is filled up with fuel, is there a way to purge it?” “No.” I then explained the numbers I had made and we went through them together.

Next step: check the fuel indicator of the plane… ~3/4… or about 80L, with the reserve deposit empty. We would be just within the limit!

We then proceeded with the preparation of the route, the pre-flight check, etc., and had good time with the flight (see report of the experience by my sister, in Spanish).

From this experience I learned a take away for future flights: when finishing your flight, it is normal etiquette towards the next pilot to refuel the aircraft if you see that the deposit is almost empty, however, it can be counter productive to fill it up completely if the next pilot is going to fly with passengers and close to the MTOW. I would then suggest that it is better to just fill it up to the volume where you know that the next pilot can have all choices open. For our DR-44 that would be filling it up to 3/4 of the main deposit (leaving reserve empty) (2). If the next pilot wants to travel along to a far distance needing more fuel he can always fill up more litres. This target weight will be different for each aircraft.

(1) See here another post I wrote two years ago about weight and balance calculations in the same plane model.

(2) Bear in mind that I found the airplane with precisely ~80L: coincidence or the previous pilot had come to the same conclusion at some point?

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Weight and balance

As part of the preparation of a flight, the pilot shall make sure that the total weight of the aircraft (including fuel and passengers) is at or below the limits, and that the center of gravity is within a certain area. This is what is called weight and balance.

Some months ago, in our private pilot licence course we had a class where we reviewed this. I remember that one pupil asked “is it possible that the aircraft gets knocked over backwards on ground?” (imagine the aircraft “sitting” on its tail).

This question led me to make the numbers to see whether this was possible with the aircraft we are using for our lessons, a Robin DR 400.

Centrogram for a DR400-120.

For this purpose we use a centrogram, which is nothing but a small “map” where the different possible weights that can be loaded into the aircraft and its momentum are already drawn so the center of gravity can be easily calculated and checked whether it falls or not in the allowed area. This centrogram is included in the flight manual of the aircraft.

I found that, carrying 2 pilots (~154kg) plus 2 passengers (~154kg), you could never load much weight into the rear compartment reserved for baggage. As that area is the one behind the main landing gear, you wouldn’t knock over the aircraft on ground.

If instead of 2 pilots and 2 passengers, you had only 1 pilot and you would load the same weight (~231kg) as baggage, the aircraft would be unbalanced but still wouldn’t fall backwards and sit on its tail. Even if the center of gravity of the baggage compartment is behind the landing gear, the weight of the empty aircraft with a center of gravity between the nose and main landing gear over compensates our trick.

Once I had calculated this, I went one step further: in which cases being under the maximum take-off weight (MTOW, 1,000 kg for a DR400-140) given by the designer could the aircraft be unbalanced? That is, what is required to get the centre of gravity out of the allowable range?

I found that to get the aircraft unbalanced basically you would need to load it in two different ways that at first seemed quite bizarre to me:

1. You would need to carry only 1 pilot and 2 passengers (plus at least 25 kg of baggage and bearing in mind not to exceed the MTOW), but instead of two of them occupying the front positions and one sitting at the back, you would have a single adult piloting the aircraft and the other two at the back. I’ve never seen that.
2. You would need to have 1 or 2 adults piloting the aircraft, no passenger at the back plus at least 100 kg of baggage at the back. Why would one person carry so much baggage for a short trip with a Robin DR 400? Here I would add that the designer of the aircraft placed a small plaque indicating that no more than 40kg should be loaded in the baggage compartment.

Well, the two cases can be referred respectively to the following situations:

1. A couple formed by future groom and bride flying alone with a pilot. Maybe that flight was used by the man to propose the marriage. Well, now you know it, that’s not a good idea, it could make the aircraft unstable (takeaway: no proposals in small aircraft).
2. Drug-trafficking. I can imagine such small aircraft being used to carry as many drugs as possible… well, apart from being illegal, if someone would carry too much (that is over 100kg), even when below the maximum allowed weight, would make it unstable.