Airliner balance question

[JammedStab]

Just reading an taxiing incident report about a 747 where they had to deplane the passengers using the most aft door.

According to the report, "When the step vehicle was properly in position at Door #5, the passengers started to disembark from the aircraft row by row at a steady pace to avoid a rapid weight shift (tail heavy situation) that would cause the aircraft to tip over on its tail."

I know that the aircraft is tail heavy on the ground with the body gear not extended but how much of a risk is there for a tip over in a normal configuration with a normal expected deplaning procedure.

Thoughts for other aircraft types as well?

[underfire]

Passengers of Walmart?

[peekay4]

Not unheard of for smaller planes & cargo planes during loading/unloading if not done correctly.

So I guess it's theoretically possible depending on the load, the slope of the ground, plus if a significant amount of passengers are jammed at the rearmost door #5. Someone can do the calculations... You probably need > 55 % mac for a 747 to tip over.

http://www.key-codes.com/images/b747_tail_tipping.jpg

A few years ago an airbridge failure at Malaga nudged a Sterling 737-800 door upwards and tipped over the plane. It doesn't take too much force...

https://www.youtube.com/watch?v=wj1xGUTcXT8

[No Fly Zone]

I think peekay4's >55% is probably ~~close, but I am NOT sure. Balance tolerances of often extremely tight. Think about this... Your flight is a few minutes late departing. You know that the cabin is turned and the FAs are ready to board 50-500 pax, but they are not doing so. And you can see the ramp rats still loading bags/cargo into the forward hold. Ahem... There may be a darn good reason. To a far lesser degree, that's also one of several reasons that front cabin pax often board before the masses of unholy and unwashed. In some locations, weather (winds) and pre push back ice loading may play a part as well. Some carriers/load planners pay more attention than others. An unplanned, forceful WHOP to the rear end can do a lot of damage and you can bet that it will always delay the departure... :8

[Metro man]

Many years ago a BA Concorde landed at Heathrow with so little fuel on board that it had to be refuelled in order to bring the nose down to the level of the aerobridge.

[Tu.114]

The ATR seems to be a bit prone to unwanted tipping. A long fuselage, a tricycle-type gear and the main door in the rear is a nasty mixture when all the passengers storm to the exit for disembarkation. I have often seen a little strut inserted below the rear fuselage for ground operations, as seen here:
http://cdn-www.airliners.net/aviation-photos/middle/3/0/2/1906203.jpg

Also, the Ilyushin 62 showed this characteristic. For loading and unloading, a retractable tail strut was installed to keep the nose wheel down.

http://cdn-www.airliners.net/aviation-photos/middle/3/5/8/1747853.jpg

[InSoMnIaC]

Many aircraft use tail stands. Including the 744

[DaveReidUK]

The Shorts 330 and 360 had a tail steady and also, as a fallback (npi) a ladder to the rear baggage compartment that engaged in slots in the door sill and would, if necessary, stop the tail from hitting the ground if the aircraft tipped when loading pax or bags (as it sometimes did).

https://upload.wikimedia.org/wikipedia/commons/thumb/4/48/British_Airways_Express_%28G-BVMY%29%2C_Dublin%2C_June_1995_%2801%29.jpg/800px-British_Airways_Express_%28G-BVMY%29%2C_Dublin%2C_June_1995_%2801%29.jpg

[Skornogr4phy]

I know it happens more often in A321's than 319's, but isn't particularly common on the narrowbody Airbuses.

[SMT Member]

The price for being really nasty in this discipline, however, goes to the MD-11 Freighter. With a dry index of around 97, and a tipping point at index 100, it takes little imagination to see who razor thin the margins are. I've seen one sit on it's tail when an A/C hose was hooked up to the inlet mounted near the nose, and set on full blast. With a bit of fuel still left in the trim tank (less than 100kg) and a couple of loaders resetting locks down the back, the puff of cold air from the A/C unit was the straw what broke the MD's back, so to speak.

747s, freighters in particular, are a bit marginal at times - particularly the -400. But not overtly so, and it's fairly easily managed.

Tail-stands should be regarded as a last-ditch effort, as an aircraft bouncing off one will usually suffer some kind of damage of the structural kind. Which is not nice at all. Cheaper than sitting the kite all the way on its tail, yes, but can still be very expensive.

[Intruder]

The 744F has a fitting under the tail specifically for a tail support stand. Either that or nosegear tiedown straps (to dedicated fittings in the concrete) are used at most stations that regularly handle the 744F.

If the forward lower cargo compartment is empty, it is easy to get a too-far-aft CG when unloading from the main cargo door.

[JammedStab]

Thanks guys,

For the 744 freighter there is a tailstand capability. However, this is not available for the pax aircraft. That is why I asked the original question about how much of an issue this really is for a deplaning out the rear door for this or other types without tailstand capability for this or other aircraft.

I hadn't really thought about it before but it seems like it may be a good idea to do what was done in the example given in the original post.

Has anybody heard of an incident resulting from a non-normal deplaning such as this.

[underfire]

probably out of abundance of caution

[NSEU]

I know that the aircraft is tail heavy on the ground with the body gear not extended but how much of a risk is there for a tip over in a normal configuration with a normal expected deplaning procedure.

It's probably a good idea to have someone monitoring the attitude of the aircraft whenever you have non-standard disembarkation or cargo unloading. Normally, on larger/longer-bodied aircraft, passenger disembarkation will be at the front of the aircraft via aerobridges with automatic level (height) adjusters (and aural warnings if the door hits the sensor placed under the open door). If passengers disembark at the rear, it will normally be via mobile steps which may not have automatic level control. Therefore, large shifts in CG can either make passenger embarkation/disembarkation difficult or damage the doors.

I've seen, on occasion, full pax version 744s almost reach full nose strut extension when too much cargo has been removed from the forward hold (from an arriving aircraft with not much fuel on board). If memory serves me correctly, the 744 electric cargo loading/unloading system will be disabled if the nose goes into "air mode" (just prior to nose actually lifting from the ground). This should at least slow down the unloading, leading to less chance of a full tilt (and hopefully make the cargo loaders run to the nearest engineer to ask them for guidance). On the other hand, passenger disembarkation is only impeded by airplane to aerobridge step size :}

[JammedStab]

If memory serves me correctly, the 744 electric cargo loading/unloading system will be disabled if the nose goes into "air mode" (just prior to nose actually lifting from the ground). This should at least slow down the unloading, leading to less chance of a full tilt (and hopefully make the cargo loaders run to the nearest engineer to ask them for guidance).

Correct, based on sensed CG I believe(assuming that it is serviceable.

Thanks for the replies and good consideration about the mobile stairs possibly causing damage to a door as there is no automatic adjustment. The door should be clear but you never know for an out of the ordinary deplaning.

[NSEU]

Correct, based on sensed CG I believe(assuming that it is serviceable.

Actually, the system to which I was referring was not the CG-sensing Weight&Balance system commonly found on freighters, but the air/ground system... which must be serviceable. When hydraulics are not powered on the 744, the air/ground system looks at nose gear strut extension only, rather than the main gear tilt sensors. The W&B system uses torque sensors in the wheel bogeys.

Back in the early days, the 744 air/ground system would shed external power completely if the aircraft sensors said the aircraft was in the air. Additional engine generator not running logic was added to prevent total power loss if the aircraft was say, jacked up by maintenance. These days, power remains on the aircraft and only certain systems go into air mode (and some busses are loadshed).

When the aircraft is tail heavy, the nose gear strut extends and the steering scissor links move closer to the strut (and in range of the air/ground prox sensors).

[Derfred]

The 737-800 becomes tail heavy about halfway through disembarking through the front door.

This is because from the overwings aft, there are still 90 odd passengers and the forward dozen seat rows are empty. Particularly if the guy at row 13 blocks the flow of passengers organising his bags, resulting in 9000kg of passenger weight aft of the main gear and zero passenger weight forward. The nose wheel oleo can lift enough to trigger a PSEU warning light (due air/ground logic), although I've never heard of one tipping.

The first time this happened to me, I called an engineer with the ground horn to query the PSEU light - he eventually came on the mike apologising for the delay because the nose was so high he couldn't reach the headset socket - admittedly he wasn't a tall fellow. :)

The effect is probably not so pronounced on twin-aisle aircraft because more passengers fit in the aisles, but for the this reason I would doubt that disembarking through the rear door of a 747 would increase the aft movement of C/G - probably the opposite.

[750XL]

The 738 and A321 are the most common culprits for tail tipping. Even during normal operations using just the L1 door a 738 can get very tail heavy if all the bags are out of H1/H2 and pax are still towards the back of the cabin.

When I used to regularly handle 737-900s we always boarded rows 1-10 first

[Anilv]

Loading is a bit tricky on the 747 Combi....

With light pax load but heavy cargo load, it may be required to hold pax on board until at least 2 maindeck pallets are off the plane. As positioning the maindeck loader is kind of tricky due to the sweepback of the wing, this may take more than a few minutes.

Alitalia had a 17 position B747 combi with pax only in zone a and b...If you were the transit station (as I was in SIN) it could be a real bitch to get in trim.

To expand a bit on the MD11, most people would be surprised that for an aircraft which was so tail-heavy empty, it was really difficult to get a fully loaded MD11 in trim as the CG would be way out the nose! Load planning would start in the warehouse where position 13C and 14C (rearmost) would be loaded /with the heaviest and densest cargo (sounds odd don't it?). Pos-14C had a limit of about 3000kgs and pos-13C was higher at around 4500-5000kgs. You'd then work on 11L/R and 12L/R (unique contours), getting these as heavy as you can.. this was made more difficult with the lesser volume compared to pos 13/14.... You'd then plan/load the lightest cargo on positions 1L/R (unique contours), this may not be always possible as accessible DGR cargo may need to be loaded on these. Pos 2-10 were normal contours so you'd load the heaviest rearmost and plan lighter pallets progresively to the front.

Sequence loading was used to ensure that the aircraft didn't tip. The sequences are as follows..

1. Starting with an empty aircraft, load the forward lower deck.
2. Load maindeck pos 1L and 1R.
3. Load pos 14c/13C but hold these pallet at around pos 7 or 8.
4. load around 5-6 pallet on the left lane. starting with pallets for positions 7L to about 12L. These pallets will temporarily occupy positions 3L to 7L.
5. Load 5-6 pallets on the right lane.
6. You now have at least 14 pallet on the maindeck and 5 in the lower deck so you can now move positions 14C and 13 C into position (they were parked midway remember?
7. Once 14C and 13C are safely locked in place you can move the right side down one at a time, feeding in more pallets as space frees up.
8. Once the right lane is loaded, you do the same for the left lane.. move the pallets in the left lane one at the time while bringing in fresh pallet as space frees up.
9. Load the aft holds.

Done...
This method is probably overkill as throughout the loading process the nose will be very heavy, an experienced loading supervisor will be able to move things along faster. A good main-deck loader operator is an asset, if he needs to keep on correcting the height to allow the loader to be level with the aircraft maindeck.. he will alert the loading supervisor.

Transit load is a bit different as you want to minimise the number of movements as much as possible.. this is where good load-planning in the warehouse pays off.

The MD11 needed several ULD contours.
1-1L
2-1R
3-2LR-10LR
4-11LR-12LR
5- 13C
6- 14C

The MD11 had another foible whereby there was a limit for the weight difference between left and right side ULDS (including lowerdeck LD3s), if you exceeded the limit your MTOW was reduced. I dont have any manuals with me so dont have the actual weights available. I understand that this was necessary to limit the sideways stress on the landing gear.

The 747F was way simpler to load..you only really needed the tail-stand if the nose door was in use at the same time.

Anil

[HamishMcBush]

Many years ago a BA Concorde landed at Heathrow with so little fuel on board that it had to be refuelled in order to bring the nose down to the level of the aerobridge.
Looking at the undercarriage layout of Concorde, I find that very hard to believe

[Amadis of Gaul]

Looking at the undercarriage layout of Concorde, I find that very hard to believe

Everything you see on the Internet (and especially on pprune) is true.