Project Management Group Case Study#6

CASe exAMPle: geneRAl AvIATIOn hAngAR ReSTORATIOn

I n t ro d u c t i o n

this case example, although for a very small project, illustrates some interesting features of planning using critical path analysis. although there are only 17 activities, the precedence logic is a little complex to draw clearly owing to the number of crossed links, and we show how this difficulty is easily overcome by inserting dummy activities at three of the crossover points.

our case example demonstrates the application of pert (program evaluation and review technique), in which the estimated duration for each activity can be subjected to a probabilistic study in an attempt to forecast the most likely completion time for the entire project.

finally, this case will demonstrate how a project manager need not accept the results of time analysis without question, but can plan to apply extra effort to expedite critical activities (usually for additional cost) to bring the planned project completion date forward

P ro j e c t B a c k g ro u n d

CEN-CONSTRUCT is a medium-sized business located in Sydney, Australia that is owned and operated by a family. It is principally a consulting company that specializes in aviation civil engineering, having worked on runway construction and paving projects as well as several other airport airside projects, most of which have been for smaller general airports. Having decided to extend its range of operations, the company recognized a local need for a contractor that could carry out aircraft hangar renovation. In consequence, the company has now achieved a good level of expertise in the interior design and renovation of hangars

the subject of this case example was the renovation of a hangar for a small client at a general aviation airport. the client uses the hangar to house his cessna 310 twin-engine plane. the hangar was originally built in 1950, since when no further construction or renovation work has been carried out. cen-construct had no previous integrated project management system or strategy for this type of project, so the project management planning steps described here were implemented for them. the planning exhibits are slightly simplified for clarity of reproduction.

P ro j e c t D e f i n i t i o n A n d A c t i v i t y L i s t

Because the total number of identified activities was small, a simple activity list (task list) sufficed for the WBS. This list, given in Figure 7.11, defines the project scope. It also defines the logical work sequence by stating the immediate predecessor(s) that must be completed before each new activity can begin. the cost estimates show that the total original estimated cost for this project was $46 900.

Readers will note that the table in Figure 7.11 contains four different duration estimates for each activity. this is because the project planner decided to use a probability assessment method known as pert to this project. this is explained in the following section.

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Flouris, T. G., & Lock, D. (2008). Aviation project management. Retrieved from http://ebookcentral.proquest.com Created from erau on 2018-09-26 17:31:09.

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A p p l i c a t i o n O f P E RT To T h e H a n g a r R e n o v a t i o n P ro j e c t

Almost all duration estimates are a matter for subjective judgement and the actual outcome for each activity can be quite different from the planner’s intention. One branch of critical path analysis recognizes this uncertainty. The method, one of the earliest critical path analysis methods, is program evaluation and review technique (pert). each person who estimates the duration of an activity is asked to give three possible values which are:

tm = the most likely estimated time to = the most optimistic estimated time tp = the most pessimistic estimated time

the values are inserted in the following formula for each activity to give the most probable or expected time, which is based on the standard deviation from a normal statistical distribution curve:

te = (to + 4tm + tp)/6.

this method was used for the hangar renovation project, and the data are given in the four estimated duration columns of figure 7.11.

Install toilet and shower

Figure 7.11 Activity list for the aircraft hangar renovation project

c h a p t e r � • p l a n n i n g t h e a v i a t i o n p r o j e c t t i m e s c a l e � � �

Flouris, T. G., & Lock, D. (2008). Aviation project management. Retrieved from http://ebookcentral.proquest.com Created from erau on 2018-09-26 17:31:09.

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T h e N e t w o r k D i a g r a m A n d I t s I n i t i a l Ti m e A n a l y s i s

the network diagram, using expected duration values, is shown in figure 7.12. time analysis has revealed the critical path (shown in bold lines) and indicates a most probable project duration of 67.9 days.

Two dummy activities, identified as A and B, have been inserted to clarify the logic drawing. These dummy activities do not affect the network logic (work sequence) and as they both have zero duration they do not affect time analysis. It is quite usual to add a start dummy and a finish dummy to network diagrams that otherwise would have multiple starts and finishes, but their use within a precedence network, as here, is less common. However, a glance at figure 7.13 shows that they can improve the clarity of a network diagram.

M e a s u re s F o r R e d u c i n g T h e P re d i c t e d Ti m e s c a l e

It often happens that the predicted timescale for a project is too long, and that was the case in this example, with the client specifying that he wanted to fly in and house his airplane between days 50 and 55 of this project. clearly the originally planned completion time of day 68 would have been far too late. there are two possible approaches to shortening the planned timescale of a project. the project manager has a choice between these two methods or, to achieve maximum expedition, can use both methods together.

Fast-tracking using concurrency Network logic often shows constraints that are not literally true, and it is known that some activities can be allowed to overlap or even run concurrently. one example is in the purchase of long-lead items, where design engineers are able to release purchasing information to the buyer before the relevant design activity has been completed. the complex constraints given in figure 7.10 showed how the precedence system will allow such overlaps to be depicted in a network. However, that process was not considered applicable on this aircraft hangar case project and will not be described further here.

Shortening activity durations by crash action It is often possible to shorten the duration of an activity by deploying more resources, using different processes, or working overtime. these actions usually add additional cost, and it would thus be wasteful to crash activities with large amounts of slack because, to do so, would have no effect on the total project duration.

Crash action must be considered first, therefore, only on activities which lie on the critical path. constructing a table that compares time saved on an activity with the relevant additional cost can theoretically produce a factor for each activity that will reveal the cost per day saved for that activity.

thus crash action can be even more selective than choosing only critical activities, because it can be concentrated on those critical activities which have the lowest cost per day saved. However, the exercise becomes complicated in practice because as one or two critical activities are crashed and the critical path is shortened, other paths through the network become critical. Crash attention must then shift to the newly created critical activities. theoretically this process can be reiterated until all activities and all possible paths through the network become critical. It could then be claimed that the network has been optimized, but that claim needs to be taken with considerable circumspection because it is based on a false premise. remember that every duration estimate is only an estimate, so the perceived mathematical precision of total network crash optimization is a myth.

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Flouris, T. G., & Lock, D. (2008). Aviation project management. Retrieved from http://ebookcentral.proquest.com Created from erau on 2018-09-26 17:31:09.

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Fi gu re 7 .1 2 n et w or k d ia gr am f or t h e h an ga r re n ov at io n p ro je ct b ef or e cr as h in g th e ti m es

Flouris, T. G., & Lock, D. (2008). Aviation project management. Retrieved from http://ebookcentral.proquest.com Created from erau on 2018-09-26 17:31:09.

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Figure 7.13 use of dummy activities in precedence networks to clarify multiple links

Install toilet and shower

Figure 7.14 Activity list for the aircraft hangar renovation project showing crashed times and extra costs

a v i a t i o n p r o j e c t m a n a g e m e n t� � �

Flouris, T. G., & Lock, D. (2008). Aviation project management. Retrieved from http://ebookcentral.proquest.com Created from erau on 2018-09-26 17:31:09.

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there is no slack in a totally crashed network, so that all or most activities are critical and that increases the risk of failing to meet the delivery date. further, the probability results from the original pert time analysis no longer apply, because each crashed estimate has been made on a deterministic, not probabilistic basis.

In the aircraft hangar renovation project a realistic approach to crash action produced the table shown in Figure 7.14 on the left. When the crashed times were substituted for the pert normal duration estimates in the network diagram they produced a total estimated project time of 43 days, well within the client’s requirements.

the notes in figure 7.14 show that some activities were less costly to crash than others in terms of the dollars budgeted for each day’s time saved.

note that although activity 10 could have been shortened from 5.2 to 3 days, the additional expenditure of crashing that task was not justified because Activity 11, running concurrently, could not be crashed below 6 days. in other words, activity 10 still had slack remaining after Activity 11 had been crashed, and there is no point in wasting money on crashing non-critical activities, because that process cannot reduce the project duration.

Summarizing, therefore, the hangar restoration project duration was originally planned as 67.9 days at a budget cost of $46 900. crash actions brought the planned duration down to 43 days but added $15 000 to the total project budget.

c h a p t e r � • p l a n n i n g t h e a v i a t i o n p r o j e c t t i m e s c a l e � � �

Flouris, T. G., & Lock, D. (2008). Aviation project management. Retrieved from http://ebookcentral.proquest.com Created from erau on 2018-09-26 17:31:09.

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