How do you rate the performance of engineers?

My current job is to manage an IT engineering department at a growing international airport. Our responsibility ranges from anything like designing a simple network to building datacenters, establishing best practices and standards and architecting complicated high availability systems.

The activities the engineers have to undertake are also broad, from drawing up standards for the cabling infrastructure to the designing various systems but also reviewing consultant designs and verifying contractor implementations.

While I realise the activities are broad and diverse all of them are performed by a relatively small team of people so each of them get involved in pretty much all areas.

In order to be able to monitor the progress, the workload and to report to upper management I am trying to work out a [KPI] system that can give an accurate representation of each engineer's load and progress, in numbers.

I've read a bit on the subject and apparently support guys are measured against the number of calls resolved and developers in terms of volume and quality of code written but the work of this team is so diverse and the outputs so different that I am yet to find a model that is representative for it.

The model should produce some meaningful representation of the work performed but also of the quality of the work. Since there is quite a bit of creativity involved in the process the numbers should reflect an exceptionally good design that is submitted late vs. a mediocre effort submitted on time. Oh, and by the way, the system should also be easy to implement in order to allow engineers to evaluate themselves without too much overhead.

What would you recommend in this scenario? How should a work package be structured to allow for such a system?

Sometimes I am feeling that I am trying to establish KPIs for poem writers. It simply would not be fair to rate them by the number of poems written.