Some days ago I saw the accident of the airtractor that, just after a drop, suffered a spar failure in both the wings. Luckily the pilot survived.

If load factor in his simplest formula is:

n=lift/weight

So if an aircraft experiences a sudden loss in weight, is natural to think that the aircraft will experience a temporary increase in G load during the maneuver. My doubt is about where that forces are going to be applied.

Let's now suppose that during the complete maneuver, the aircraft is going to maintain straight and level flight without losing or gaining a feet of altitude. In this case, will the complete aircraft be under that load (like during a turn), or the load is going to be applied only on the wing(s).

• If the load factor > 1 then the aircraft is not in straight and level flight. Dec 12, 2021 at 20:12
• As stated in the Consolidation and Analysis of Loading Data in Firefighting Operations of the FAA: The effect of relatively large aircraft weight changes due to the release of retardant causes uncertainty to the assessment of ongoing structural integrity, since mission weights have not been routinely tracked. Dec 12, 2021 at 22:48
• Preliminary evidence obtained from some recently instrumented aircraft suggests that the high g levels occur both at high gross weight prior to the retardant drop and at lesser gross weights after the retardant drop. The overall rate of damage accrual determined from measured acceleration data should account for representative changes of aircraft weight in a realistic or conservative manner. tc.faa.gov/its/worldpac/techrpt/ar05-35.pdf Dec 12, 2021 at 22:50

There are a couple of misconceptions in your question that I believe are leading you to a false conclusion that G-loads from releasing payload may have literally ripped the wings off this airplane:

1. Spraying liquid will not cause a "sudden" loss of weight. While fire fighting aircraft are capable of releasing large quantities of liquid all at once, agricultural sprayers like the one depicted are designed to disperse over a much larger area. Hence the gross weight lost on each pass is pretty negligible.

2. Losing gross weight will not cause a sudden and appreciable increase in load factor. Whatever the math might say, I can tell you from the personal experience of standing in the cargo bay of C-130s as thousands of pounds have rolled out the back end, that there is very little perceptible change in felt G forces. (Conversely, I have been walking forward in the cargo bay when the pilot put on a roughly 2G turn and my knees buckled, sending me to the deck and breaking a tooth...)

So, while load factor will change somewhat when weight is changed suddenly, it would be well within structural limits that the aircraft is designed for, and would in no way account for the type of dramatic spar failure as depicted in the photo.

If you are interested in a similar incident, a C-130 tanker tragically and dramatically had its wings snap off in a similar manner. More info in the link here: C-130 crash

I don't recall all the details of this, or the incident you describe, but generally there is corrosion, and/or fatigue cracking, and when the catastrophic failure finally occurs it is as a result of the pilot applying G loads during the pullout, not because of the gross weight change from dropping payload.

• Thanks for your answer Michael! It helps a bit to solve my doubt; probably most of the load is going to be applied only on the wings/torsion box of the aircraft. The air tractor in the picture was not damaged during spraying but during a complete release of water, you can find the video here: instagram.com/p/CXWzus1FyEW The aircraft structure was probably compromised well before the accident. Dec 12, 2021 at 23:56
• @Spitfire01, thanks for the video link! You can definitely see that the pilot starts a pretty good pull just before the wings snap. Amazing that he survived... Dec 13, 2021 at 0:19
• Just dumping load increases the G force you feel inside the plane because the same amount of lift produced by the wings. What changes is the amount of downward force gravity exerts on the plane + stuff inside it, because there's less stuff. So if speed and pitch stayed the same, you'd expect the wing load in terms of force in Newtons to stay nearly constant, even though people in the plane briefly feel an upward acceleration. Changing the weight distribution probably will lead to some pitch change, though, and of course the pilot pulling up will be a factor. (@Spitfire01) Dec 13, 2021 at 3:41
• TL:DR: my point is that even if you did feel an acceleration while standing in the back as the load drops, that doesn't mean there's more force on the wings. Just that the same lift is now opposed by less weight. Dec 13, 2021 at 3:42
• Related to quantity you're right in general indeed, but this case is different: "pesticide application was canceled due to bad weather conditions. The pilot discharged water from the tank onto the landing site and began the climb and his wings broke and the aircraft hit the ground.". The tank was emptied at once. Maybe this was not so much water, we don't know.
– mins
Dec 13, 2021 at 15:20

Your skepticism is justified, and it's not just a question of how significant the change in G-load created by dropping a payload would be. Dropping weight from the fuselage creates no increase in the stress on the wing-fuselage junctions. In fact, dropping payload from the fuselage decreases the stress on the wing-fuselage junctions. (Dropping weight from the wings, however, would increase the stress on the wing-fuselage junctions.)

Assume an aircraft weighs 10,0000 pounds without payload, and can carry a 10,000 pound payload. Assume the weight of wings is negligible, so the fuselage weighs 10,000 pounds without payload. Assume the payload is carried in the fuselage.

In straight-and-level flight with no payload, 10,000 pounds of force must be exerted on the fuselage by the wings. So 5000 pounds of force must be transferred through each wing-fuselage junction.

In straight and level flight with payload, 20,000 pounds of force must be exerted on the fuselage by the wings. So 10,000 pounds of force must be transferred through each wing-fuselage junction.

Assume the plane is in straight-and-level flight with payload, and then the payload is dropped instantly. The weight of the aircraft is cut in half. The airspeed and angle-of-attack of the wings will initially stay the same, so each wing is still generating 10,000 pounds of force, all of which is transferred through the wing-fuselage junction. There is no increase on the stress on the wing-fuselage junction. The aircraft is now in a +2 G condition and will accelerate upwards. I.e. the flight path will curve upwards. This upward acceleration will create stress on the mounting points of heavy components such as the engine, battery, pilot's seat (including pilot), etc, but will not increase stress on the wing-fuselage junction.1

The upward acceleration will have some tendency to decrease the angle-of-attack of the wings, decreasing the total lift force, and decreasing the total amount of force transferred through the wing-fuselage junctions, and decreasing the total G-loading, but this will be offset by the plane's inherent pitch stability dynamics (assuming that the payload was carried at the CG), which will tend to pitch the nose up to conform to the upward curve in the flight path and restore the original angle-of-attack.

At any rate, we can see that there is no tendency for the wing-fuselage junctions to fail in a way that would cause the wings to rise upwards relative to the fuselage. The same amount of force is being transferred from the wings to the fuselage as was the case before the drop, but that force is now being used to accelerate the fuselage upwards rather than to support the payload.

Now assume that the weight of the wings is not negligible. Assume the wings weigh 500 pounds each and the fuselage weighs 9,000 pounds without payload. Now in straight-and-level flight with a 10,000 pound payload in the fuselage, 19,000 pounds of force must be exerted on the fuselage by the wings. So 9,500 pounds of force must be transferred through each wing-fuselage junction.

Now when the payload is dropped instantly from straight-and-level flight, and the wings continue to fly at the same airspeed and angle-of-attack and generate 10,000 pounds of lift each, the aircraft will be in a 2-G condition and will accelerate upwards. 1000 pounds of lift force will be "absorbed" by each wing, and 9000 pounds of lift force will be transferred through each of the two wing-fuselage junctions to the fuselage. So dropping the payload from the fuselage has actually decreased the stress on the wing-fuselage junctions, because some of the lift force from the wings is being used to accelerate the wings upwards, rather than to support the payload in the fuselage!

Now let's change the picture and assume that the payload is carried in the wings, not the fuselage. Let's assume the wings weigh 500 pounds each empty, and 5,500 pounds each loaded. Let's assume the fuselage weighs 9,000 pounds. In straight-and-level flight in the loaded condition, the wings are generating 20,000 pounds of lift total, of which 9,000 pounds are being used to lift the fuselage. 4,500 pounds of force must be transferred through each of the two wing-fuselage junctions.

Again, assume the plane is in straight-and-level flight with payload, and then the payload is dropped instantly. The weight of the aircraft is cut in half. The airspeed and angle-of-attack of the wings will initially stay the same, so each wing is still generating 10,000 pounds of force, so the aircraft is accelerating upwards in a 2G condition. Of the 10,000 pounds of force being generated by each wing, 1000 pounds is "absorbed by" the wing and 9000 pounds must be transferred through each of the two wing-fuselage junction to the fuselage. The force on each of the wing-fuselage junctions has doubled.

Take-home lessons--

• Dropping weight from the fuselage does not increase the stress on the wing-fuselage junctions. Rather, if the weight of the wings is not negligible, then dropping weight from the fuselage decreases the stress on the wing-fuselage junctions as the aircraft accelerates upwards.

• Dropping weight from the wings does increase the stress on the wing-fuselage junctions as the aircraft accelerates upwards.