I wonder what the compensation packages look like for these roles. What does coming out of retirement for an assumed well paying job for up to a couple of years??? do to someone's retirement financial plans? What if you're on a fixed income from the gov't, and now you have a salary again? Is paying them under the table viable?
The financial benefits are usually quite extensive for the engineers involved, because there is nowhere else to obtain supply of the needed skills.
Assuming they are in the US, and the fixed income they are on is Social Security (SS), what happens if you have income other than SS is that it can change how much of your SS income is taxable, and if you are under your full retirement age it may reduce your benefit (which will return to normal when the job is over).
For taxing the key is a thing called your "combined income" (CI), which equals your adjusted gross income (AGI) + tax free interest income + 1/2 of your SS income. (NOTE: SS income is not part of AGI).
For individuals if CI <= $25000 then SS income is tax free. If CI > $25000 some of your SS income may be taxable. For couples filing jointly SS income becomes partly taxable when CI > $34000.
For people who retired early, if the job pays well enough once you go back to retirement you could find that your SS benefit is higher because the SS taxes on the job earnings go into the benefit calculation so you might come out ahead even if your monthly benefit was temporarily reduced during the job.
I think those are the main considerations if you are on SS and considering taking a high paying job.