Wood and Sweginnis (1995) consider that the use of any formal analytical technique is better than an unstructured approach to an investigation. All formal techniques have the following advantages over an unstructured approach:
• They form a framework around which to organise the data as it is collected
• Missing links can be identified and sought
• Integration of team efforts, in a larger investigation, is easier because a model of the potential accident sequences shows how evidence supports or rebuts hypotheses
• Report writing is simplified when a model of the accident sequence is available as a guide
• A picture of the sequence of events is a powerful aid to communication, especially in support of recommendations.
Wood and Sweginnis (1995) divide formal analytical techniques into two broad groups: Data Management methods, and Cause Resolution methods. There is also an intermediate stage, where ‘barriers’ or ‘defences’ are considered without analysis of causation. These groups are discussed below.
Data management methods correlate data from different sources against some baseline, so that the interactions of various events can be seen. They are useful to explain the sequence of events in a complex accident. The baseline can be the aircraft's flightpath, or a timebase (Wood and Sweginnis, 1995).
Flight Path Diagrams.
Flight path diagrams are frequently used in reports. Various factors (including time) are annotated along the aircraft's flight path, as constructed from recorded data or witness evidence (Wood and Sweginnis, 1995). The flight path diagram from the report of the accident to ZK-NZP (OAAI, 1980) is shown in Figures 7a and 7b.
Matrix Analysis.
Matrix analysis is a method of resolving uncertainty generated by apparently conflicting data, such as witness statements about the same event. Observations of events are arrayed by observer, and in sequence, so that observations of the same events are in proximity. This enables discrepancies to be seen, so that the causes of discrepancies may be accounted for. If exact time is available, a timebase could be used.
Sequentially Timed Events Plotting.
In Sequentially Timed Events Plotting (STEP), all known events are plotted against time (Hendrick and Benner, 1987). When possible, the time taken for an event (for example a radio communication) is also shown. STEP is similar in function to flight path plotting, but the baseline is time. However, Hendrick and Benner (1987) take the view that the use of causal linkages in STEP, and the 'necessary and sufficient' test for logical validity of entities, raise STEP into the category of cause resolution methods, discussed later.
Figure 7a. First part of Flight Path Diagram. (Source: OAAI. (1980). Air New Zealand McDonnell Douglas DC10-30 ZK-NZP Ross Island, Antarctica (Aircraft accident report No. 79-139). Wellington: Government Printer.). Key:
A. I’ll have to do an orbit here, I think
B. Well actually its clear out here if you can get down C. It’s not clear on the right hand side here
D. We’d like further descent or we could orbit in our present position which is approximately 43 miles North descending VMC
E. I’ll do an orbit here to get down, I think
F. We are presently descending through Flight Level 130 VMC and the intention at the moment is to descend to 10,000
G. Transponder now responding H. We’ve lost him again
I. You’re through 10,000, are you going to hold it here? J. I’ve got to stay VMC here, so I’ll be doing another orbit
K. We’re maintaining 10,000 presently 34 miles to the North of McMurdo
L. Still negative contact on VHF we’re VMC – we’d like to let down on a grid of 180 and proceed visually to McMurdo
M. Well we’re just going VMC to McMurdo and then come back in N. We’re VMC around this way so I’m going to do another turn in O. You can’t talk if you can’t see anything
P. There you go, there’s some land ahead Q. I’ll arm the Nav again
Figure 7b Second part of Flight Path Diagram. (Source: OAAI. (1980). Air New Zealand McDonnell Douglas DC10-30 ZK-NZP Ross Island, Antarctica (Aircraft accident report No. 79-139). Wellington: Government Printer.). Key:
S. We’re now at 6000 descending to 2000 and we’re VMC
T. We had a message from the Wright Valley and they’re clear over there U. The Taylor or the Wright now or do ya/ No – I prefer here first
V. I still can’t see very much at the moment – as soon as I see something that gives me a clue as to where we are I’ll let you know
W. Altimeters X. 29 – 29.30
Y. Alt, Nav Track, Vert Speed
Z. Where’s Erebus in relation to us at the moment?/ Left about twenty or twenty five miles, I think/ Left do, ya reckon (?) Well I don’t know I think (?) I’ve been looking for it?) I think it’ll be (?) I’m just thinking of any high ground in the area, that’s all
AA.That’s the edge
BB.We might have to pop down to 1500 here, I think/ Probably see further in anyway CC.Bit thick here, eh (Bert)? / Yes my xxxx oath!
DD.You’re really a long time on xxx instruments at this time, aren’t you?
FF. Alt Hold
GG.Actually these conditions don’t look very good at all, do they?/ No they don’t HH.That looks like the edge of Ross is there
II. I don’t like this
JJ. We’re 26 miles North, I’ll have to climb out of this
KK.It’s clear to the right, and (well) ahead – you’re clear to turn right, there’s no high ground if you do a 180
LL.(GPWS) Woop-woop pull up woop-woop MM. 500 feet.
NN.(GPWS) Pull up OO.400 feet
PP. Woop-woop pull up woop-woop QQ.Go-around power, please