[SystemSafety] Miss vs Ms

[Grazebrook, Alvery]

I think you are missing a very important point. Clearly, in this particular
case there was not an accident - not even close. However, there are cases
where this could be a contribution to an accident. One of the reasons that
the aviation sector is much safer than some other forms of transport is
that problems like this are systematically addressed and taken seriously.

Almost as an aside, the absolute weight is one question, and weight
distribution is another. A poor weight distribution can cause an aircraft
to become uncontrollable. The weight distribution is typically expressed as
%MAC. To over-simplify, an aircraft might be acceptably controllable in the
range 15% to 40% MAC (longitudinally). 50% MAC is where the centre of lift
and the centre of mass are in the same location - aft of this the aircraft
is unstable. Fuel burn might cause a swing of say 5% MAC either way in
normal operations. So, on my oversimplified aircraft, the safe loading
envelope is between 20% and 35%. For an aircraft of the type discussed, MAC
length appears to be in the range 3.5 to 4m. So shifting 1T by 1m on a 66T
aircraft would have close to 0.4% MAC change, but the cabin is around 20m
long, so shifting this much weight from one end to the other makes around a
8% MAC difference. This could shift the aircraft from safe to completely
un-flyable. The problem is worse on smaller aircraft as you suggest. Weight
and weight distribution is a big deal in flight safety.

The corollary of this is you should never recommend a rugby team to run
lengths of a flying aircraft.

Cheers,

Alvery

*** the opinions expressed here are my own, not necessarily those of my
employer.

On Mon, 12 Apr 2021 at 04:31, Eric Burger <eric.burger at georgetown.edu>
wrote:

> Depends on the regulatory regime. In the USA, the FAA realizes that the
> smaller the aircraft the higher the probability of getting unlucky on the
> distribution. Same in the UK. With larger numbers and larger aircraft,
> getting the statistical child wrong has much less impact.
>
> Note this was an IT failure, per the AAIB Bulletin. A FAA Statistical
> Child (82#) is less than half the weight of a Statistical Adult Female
> (179#). The UK’s numbers are 35 Kg / 60 Kg. However, the aircraft in
> question counts as a large aircraft by the FAA definition (187 passengers
> is much more than the cutoff of 71). It is possible that all of the Misses
> were sitting together in the front on one side, but even then, we are
> talking an error of 1200 Kg on a flight loaded to 66495 Kg (the discrepancy
> is slightly less insignificant, but still insignificant, as the zero fuel
> weight was calculated as 56,716, not the expected 57,916. I realize that
> when one is on the edge of W&B, a 2% error can be really bad, but this
> flight was nowhere near that (in this case, 61,688 Kg). The impact? The
> takeoff V numbers were off by one knot (0.7%). I’d bet dollars to donuts
> that would go unnoticed - anyone want to calculate how many feet that adds
> to the takeoff roll on a 737 that is accelerating? Would the pilot notice
> the plane was microscopically taking longer to hit V1?
>
> Now, if this was an Embraer E170 (~65 passengers) that error would be bad.
> For an ATR 42 (~40 passengers), that error could be close to catastrophic.
>
> > On Apr 8, 2021, at 12:26 PM, Hugues Bonnin <hugues.bonnin at free.fr>
> wrote:
> >
> > Ok, it’s obvious that weight is critical, my remark is not on that. My
> remark is on the deduction of the weight from the categorisation
> adult/child only ; as a group of child can be heavier than a group of
> adult, it seems questionable to base critical element in this
> categorisation.
> >
> > Regards
> >
> > Hugues
> >
> >> Le 8 avr. 2021 à 16:55, Peter Bernard Ladkin <ladkin at causalis.com> a
> écrit :
> >>
> >> 
> >>
> >>> On 2021-04-08 15:41 , Gareth Lock wrote:
> >>> Hugues,
> >>> The issue I can see would be when you have a smaller aircraft and
> therefore the impact of mass would be greater.
> >>
> >>> *From: *systemsafety <
> systemsafety-bounces at lists.techfak.uni-bielefeld.de> on behalf of <
> hugues.bonnin at free.fr>
> >>> IMHO, I don't see how this difference could lead to serious problems,
> >>
> >> 2.2% of the ZFW. Sure that can lead to problems, such as calculating TO
> thrust and balanced field length.
> >>
> >> A weight mismatch can also lead to balance problems, depending on the
> seating algorithm used by the airline, but in a 187-passenger aircraft I
> would expect not.
> >>
> >> PBL
> >>
> >> Prof. Peter Bernard Ladkin, Bielefeld, Germany
> >> ClaireTheWhiteRabbit RIP
> >> Tel+msg +49 (0)521 880 7319  www.rvs-bi.de
> >>
> >>
> >>
> >>
> >>
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