Monday, March 08, 2010


When you put cargo in an airplane, it needs to be tied down. Not so much so it doesn't fall over and break, but so that it doesn't move around and kill you, either by falling on you or by shifting the centre of gravity such that the airplane becomes unflyable. And kill you. Securing the load in an airplane is important.

When the load is people, the tiedowns are called seatbelts. Often the tiedowns are seatbelts when the load is not people, too, because seatbelts are easy to fasten, easy to adjust and certified for holding objects upwards of 200lbs in place. Humans don't usually have 90 degree edges on them, and objects don't usually have waists, so using seatbelts is discouraged for non-human objects. If your load consists entirely of non-human objects, then the seats and attached seatbelts can be removed from the equation (and airplane) entirely and dumped in the back of the hangar before the cargo is loaded and secured to tie-down rings.

A removable cargo tie-down ring consists of a square of metal with a ring attached to the top. It's about the size of a stack of seven saltine crackers, with a ritz cracker balanced edgewise on top. There's a channel in the bottom of the metal square so that it can slide onto the same rail that an aircraft seat attaches to, and a thumbscrew on the side to secure it in position once it has been slid to the desired location. Each tie-down ring, of course, costs over $200, about ten times more than a piece of metal with a ring and a thumbscrew on it ought to.

The law of tie-down rings is that there are never enough. One reason that there are never enough stems from the fact that there are often not enough, so captains learn to hoard them in their flight bags, exacerbating the shortage, and thus the hoarding. But even if you have a whole flight bag full of tie-down rings, you still can't secure your cargo properly because you can't put them in the right places.

You can only put the removable ones where there are seat rails. In other places you have to rely on the ones the manufacturer provided. You're lucky if there are any tie-down rings installed in the cargo compartment. I have worked with people who have resorted to the "pack it so tightly it can't shift" method. I suppose if you have literally packed the aircraft floor to ceiling with similar density cargo, and there is protection to prevent them from entering the cockpit, and you have forward emergency exits, you're covered, but just packing things in the rear cargo tightly enough that they are hard to remove won't cut it. Normal vibration or abnormal deceleration could still bring them down on top of you.

You have to plan the packing so you get the tie-down straps hooked into the tie-down rings before you bury them in cargo, and then just tighten everything down. The ratcheting cargo straps are nice, but I hate it when they get all jammed up in the reel.

Once I worked at a place that had truly awesome cargo nets that secured into custom recessed ports all around the inside of the cargo space. One airplane still had mismatched parts on the empennage from repairs following a years-ago towing accident, but the customers were safe from their cargo and the pilots could work efficiently. Rare that a company gets that priority right.


X-av8r said...

I liked your description of tie-downs, using crackers. I can just see them. You must have been hungery when that came to mind.

Years ago a good friend of mine was a flight engineer on an Air America C-130 flying an early morning take-off out of a base in Texas, bound for South America. On rotation, the aircraft reared up and climbed straight up and did a hammerhead stall into a hangar beside the runway. Everything and everyone was burned to a crisp. They think that the cargo, sitting on a roller-conveyer floor was not secured properly, broke loose and slid to the tail upon rotation.

At one time I was a Loadmaster in the USAF and we were always concerned with the positioning of the cargo for weight and balance calculations, and the strength of the tie-down straps and rings. Another thing we had to have in mind was a jettison plan in case of a multi-engine failure. We could open the rear cargo doors and dump everything into the ocean, if it meant saving the airplane. Loadmasters carried large sheath-knives for quickly cutting cargo straps, if required.

Aluwings said...

Human cargo has been known to shift and cause problems too. Like the time the DC9 suddenly pitched up almost uncontrollably at lift off. The W&B calcs were all done properly, but no one warned the pilots that the entire football team had decided to ignore their assigned seats to be together right at the back of the plane. Ooops.

Anonymous said...

X-av8r: I'm having trouble imagining how you could manage to cut the cargo straps and have the cargo slide out the rear door without risking you also sliding out the rear door. I assume there is some friction that would tend to hold the cargo in place unless the airplane is noticeably pitched up. Of course if the alternative is the airplane will soon be in the water, maybe you would be well-motivated to find a way.

Also, as you let the aftmost cargo out, wouldn't that shift the balance forward, making it harder to keep the nose up and make it difficult to get the forward cargo to slide out?

verification word: ingizata -- the crisis-inspired ingenuity to get it out of there

Aviatrix said...

X-av8r: That's a horrific image. What kind of tie-down straps and securing points do you use keep SUVs from shifting?

Aluwings: I didn't know that one. My story for that is from an airplane evacuating children from East Berlin. The pilots had an aft CG shift due to the children's governess lining them all up at the rear lav before landing.

Anonymous: Personnel working in the vicinity of an open door on an aircraft generally have safety harnesses clipped to overhead lines. If you watch a military movie, you can probably see the lines in the obligatory scene at the door of the airplane.

5400AirportRdSouth said...

When I worked on the ramp for FedEx, one of our jobs was to inspect the cargo tie-downs and container/pallet locks built into the floor of the cargo compartments.

Two people did individual inspections of each aircraft and signed off, with witness signatures before loading could begin, that all locks were functional. I think you were allowed two locks total on the main deck to be U/S and only one U/S lock per position.

The 9G netting that separates the cargo area from the forward/flight deck area on converted airliners is impressive as well. Not only is it rated to take an impressive load on it's own, it also has built-in " shear strips " to allow it to progressively "rip" and dissipate energy.

nec Timide said...

Yes, you can never have too many tie down rings. Years of flying and owning pickups and SUVs have taught me that lesson well.

I was quite pleased when I was looking at my Cherokee pre-purchase that it has two sets of very beefy straps bolted to the cargo area floor. Not so pleased that the short end is away from the door requiring me to either do the stance of the cobra through the cargo door, or climb in the back seat to properly secure. But there is no substitute for knowing that sudden turbulence or rapid deceleration isn't going to result in having Samsonite stamped in the back of your head.

Rings said...

I know that when i was in the Marine Corps, the load master was GOD on the flight deck. It was their way or the highway, I guess for exactly what X-av8r was saying...

If that load shifts, it is all over..

X-av8r said...

The Air Force, when I was there, used canvas straps rated at 1500 lbs and 2500 lbs, both with ratcheting lever hooks that we would secure to flip-up rings in the floor. Heavier cargo was tied down with 15,000lb chains or 25,000lb chains with threaded tighteners fastened to the floor with big screw-in floor rings. most of the really heavy cargo, like aircraft engines, were in shipping containers with tie down points built into them. Passengers always road in temporary seats aft of the cargo and all seats always faced aft. Ridding backward was consider safer in case of a real solid deceleration. Sometimes we configured the aircraft with troop seating where they sat sideways.

A jettison plan was a best concept of how to get rid of a lot of weight in a hurry. We had an elaborate overhead hoist system that could be run all the way aft, then drag whole pallets to the rear door and levered out. Part of the plan was the sequential moving of cargo so that the aircraft would be flyable during all this. If they were too heavy to be dumped, they were to end up at a satisfactory CG with every chain on it that we had. In the case of an air-drop we used a roller conveyor floor and the whole thing all zipped out at once.

One of the oddest things we used to load was nuclear sub periscopes in their shipping cases. they were about 65 feet long and a bit of a challange to lever into the cargo compartment.

Aluwings said...

re: "A jettison plan was a best concept of how to get rid of a lot of weight in a hurry"

Hopefully that didn't include any passengers! Yikes - think I'd wait for the non-cargo flight ;-)

Capt. Schmoe said...

Riding in the cockpit of a C-17 with four trucks, two pallets and a tractor in the cargo bay. Pilot opts for a TAC descent (about 9000 fpm I think)and the AI is almost all brown. All I can think about are those trucks breaking loose and pushing us all out of the front of the aircraft on the way down.

They do it all of the time though and once I realized things were good, it was a blast! They used beefy chains and hard points on the floor.

Traveller said...

Current port dawg.

My job is to prepare the cargo for transport and to secure it correctly in the aircraft. Loadmaster has final say on the security of the cargo. Some cargo is better suited to transport than others. Sometimes the problem is not enough hard points of the cargo. Also, as noted in a couple of the OP reports, there is a proper way to restrain cargo like the meat mentioned in the one incident report.

My favorite tale of mis-loaded cargo is from the Berlin Airlift. A DC-3 was fully loaded with was was manifested as corrugated sheet aluminum. On take-off, full power gets the plane off the ground but is required to maintain tree-top level for the flight. The main gear collapsed on landing. After action report reveals the cargo to be sheet steel. A testament to the strength of the DC-3 and the importance of verifying the load.