[SystemSafety] Miss vs Ms

[Eric Burger]

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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