Guest Post by Ajlr
Staying in Trim
Ever wondered how the captain of a big loaded cargo or passenger aircraft knows that it will be able to take off – that is, that the lift given by the wings and other aerodynamic surfaces at take-off speed is greater than the force of gravity holding it on the ground? Or why it doesn’t either topple onto its nose, or one wing, or drag its tail in the dust? It’s all to do with the payload and ‘trim’ – and by that I don’t mean what colour the seat cushions are.
Many years ago I trained as an air movements officer, a branch of (military) air transport dealing with the movement of passengers and cargo up and down the transport routes to the Far East, Germany, North and (occasionally) South, America, that the RAF operated in those days. It was fascinating work and I enjoyed it a lot. To get to the point of being a qualified ‘mover’, however, officers (and senior NCOs) had first successfully to complete a four month specialist course. At the end of the course, the test was for the group to select and load – and then fly with – a cargo to a base somewhere down-route. That sort of practical test tends to concentrate the mind wonderfully!
The course itself took us steadily through how to calculate what the available payload for any given aircraft might be, and what the local conditions might be that would affect that payload – ie, if you knew that the aircraft was scheduled to be taking off in Cyprus at noon in the middle of summer, then the real payload would be less than if one was taking off at midnight in an English January. High temperatures = ‘thinner’ air = less lift from the wings = less payload available. The fuel load is also adjusted according to these conditions. Then there were ‘bending moments’ and ‘shear’ to watch out for – if you’re loading something into a long tube (which is what an aircraft is) that’s supported mainly through midsection points – ie the wings – then you really (definitely!) want to avoid the ‘tube’ bending at either or both ends due to an excess weight of cargo there. The ‘small furry mammal youngster being lifted by the scruff of the neck’ silhouette is so not something one looks to find in an aircraft! Nor do you want ‘shear’ force to come into play, where the absolute weight in a particular section may cause it suddenly to force its way through the floor and down (possibly a loooooong way….) out of the middle of the tube. These are the things to keep any mover awake and sweating – and focussed – on a long shift in the dark hours!
After the considerations above have been borne in mind, one of the next things to work out is restraint (of the freight, that is, not the movements officer) so that the various g-forces (up, down, and sideways) liable to operate in flight don’t cause pieces of cargo to tear loose and go looking for squishable (human or other) company mid-flight. Nets, straps, and tensioners to hold everything in place all get used. And in those days, before the advent of much pre-palletised cargo, one also had to consider ‘floor loading intensity’. Aircraft floors are strong, but not as strong as a solid floor made of something like concrete (one doesn’t get concrete floors in anything that has to get off the ground!). So, just as a stiletto heel can puncture or mark even hard surfaces, an aircraft floor can be badly damaged if something heavy with a particularly small footprint is put directly on it. FLI capabilities for particular cargo aircraft decks are all known, so one compares the FLI of the cargo item with that of the aircraft one is loading, and if it exceeds the limits then one puts a calculated number of load spreaders (usually solid pieces of flat timber of correct thickness and area) between cargo and floor.
Of course, at the same time as calculating the safe distribution of cargo throughout the aircraft one also has to keep in mind the itinerary. One does not, for example, want to load something large and complicated at the back near the cargo hatch (even if it balances the load beautifully) if it’s heading for somewhere right at the end of the route and will make unloading and loading items due at intermediate stops “almost s*dding impossible so they’d have to take the b*st*rd out each time” to quote one of our instructors… What a colourful turn of phrase those men had, to be sure.
As you can imagine, I and my fellow course members spent many a spellbound hour crouched over practice trim sheets – the diagrammatic representation of the fuselage and load allowing calculation of all the essential elements – sweat bedewing our brows as we strove to find the right mix of cargo for particular journeys. To think that we would all be responsible for leading/managing teams of movers once we had our postings at the end of the course! OK, the captain of the aircraft would always go through the trim sheet with the duty movements officer before take-off and if he (it was always he, in those days) wasn’t happy with it then Things Had to be Changed. But no mover wants that sort of egg on their face.
And then, of course, there were the horrors of human passengers to cope with as well. OK, easy to calculate their load (180 lbs per human, whether man, woman, or child) but this type of sentient cargo could be troublesome. For some reason they seemed not to want to be tied down in nice neat rows on the floor. No, they wanted ‘seats’! They even wanted to be told what was going on! (Oh the horror that was the Washington-bound flight on Wednesday and Friday mornings, full of people with gold rings up their sleeves…!) A few people moving around the aircraft during the flight wouldn’t make much if any difference in a load that was all passengers, but in one of the smaller aircraft one wouldn’t want the entire passenger list to rush for the back loo at the same moment. Luckily, the airloadmaster/mistress (one of the aircrew) could always be relied on to nip any such behaviour off at the start. A favourite suggestion offered to troublesome children on some flights was the muttered advice to ‘go and play outside’…
Anyway, after a few weeks of theory we came to the practical side of things. I and my dozen fellow course members were let loose on various mock-up fuselages and encouraged to play with different arrangements. We crawled around on the floor, fastening everything movable to all things immovable, against theoretically possible but practically improbable g-forces. One day we had a real live vehicle to play with as well, as part of the cargo. Guess who was encouraged to be the one crawling under it (“but you’re the smallest”) to fasten the axles down to the strops? Guess who then found that grinning colleagues had quickly fastened straps right round the edge so that it was impossible to crawl out again! It was amazing, though, how quickly those straps were unfastened once I’d reached out and banged a few handy shins with a couple of spare pieces of timber. That and some clearly ennunciated threats…
Came the day when we had our real payload to handle at the end of the course and we found we were going to be popping over to one of the bases in Germany with it. Well, that wasn’t too bad, only 90 minutes’ flight each way and we were being allowed one of the nice easy-to-load Hercules to do it with. Of course, that 90 minutes was going to be at the end of a 30-hour shift – once ‘our’ aircraft was available for loading we had to select the cargo, load it, process it, fly with it…there was one particular group of equipment items I practically felt related to, by the end of that time.*
Moving cargo from the freight hangar to the aircraft is done by all manner of means – in the back of landrovers, in trailers, or – often – on a vehicle called a Condec Transfer Loader. This was basically a motorised loading platform, that could be raised (to the height of an aircraft loading ramp) and lowered (to allow walk-on/drive-on loading and unloading of items). In most active teams of movers at that time there were around 15 – 20 other ranks, plus the movements officer. Driving the Condec was always a favourite job – who wouldn’t want to be the one sitting at ease there, while everyone else strained ligaments. There was a risky little habit of one team of movers, though, to put the edge of the Condec platform just underneath the edge of the aircraft’s loading ramp, to support it as the load went over – it saved time on installing the ramp’s ground support struts, as should happen. (I well remember witnessing, about a year later, the uproar when a Condec being used in such a way to transfer a heavy load across the ramp suddenly lost its hydraulics and the aircraft – a Belfast – sloooowly started to tip up at the front and down at the back onto its tail. My, the shouting! That practice promptly stopped.)
What no-one had bargained for with our load was that by the time take-off time came round, the weather had gone sour on us. The captain duly inspected and signed our trim sheet, to confirm that he’d be happy to take the aircraft into the air with what we’d loaded on it, but then informed our red-eyed-but-jumpy-with-adrenaline group that we’d need to hang on once we were up because there were likely to be some funny old weather systems around that night. He was right, too (captains are always right). At first, dropping in a series of little 200 foot air pockets seemed like hilarious fun – helloooo Mr Ceiling! After a while….well, no, I’ve played more interesting games. I’ve never much liked flying in the back of aircraft anyway when it’s so much more interesting at the front. That night, I decided that strapping myself in to my seat and dozing was really much more sensible and decorous. Not to mention that after more than 24 hours in those overalls and boots I probably wouldn’t have won any prizes for ‘most well-groomed and fragrant person’! Landing at our destination – even the swooping, almost-sideways kangaroo hops we came down with in a fierce cross-wind, was a considerable relief. At last we could count ourselves as movers – and get some sleep!
* * *
* This was part of the test at the end of the course. Normally, movements teams at a transport base worked shifts of no more than 12 hours, handling maybe five or six aircraft in that time. Some of the mobile air movements teams, though, could and did work very long hours at the strange destinations and with the strange loads they handled.
comments
Please join the discussion at Robin McKinley's Web Forum.