I have a few clients for whom I have to spend a few hours per month on a plane. Should I be charging for travel time?
This is truly something that should be worked out in your consulting agreement before the project starts. If you are working during travel, then this question is moot: you are working on behalf of your client so you can legitimately charge for hours worked. This will likely be at full rate if you believe you are working at full productivity.
I presume, however, you are referring to time you are not working for a client but are prevented from billing any other client. If you take a morning flight of several hours, then half a potentially billable day is lost. On one hand, you provided no services to a client so the client might claim you should not be paid. On the other hand, the travel was needed for you to be on site to provide services you couldn't otherwise provide off site. Recognize that a hard line on either position is likely to create some tension.
How you resolve this depends on how well your client trusts you and how much travel involved. If your client is generous, then travel both ways might be chargeable. However, a fair approach for most clients is to split the difference and pay for half the travel, assuming you are traveling and not working for another client or on your own firm's administrative duties during that time. Tip:
Talk to your client and come to an equitable arrangement. Explain your rationale and why you are willing to share the cost (since you are incurring real costs). However, make absolutely sure that you are being ethical and not being paid to travel by one client and charging another for work you are doing on the plane - double billing for the same time. © 2011 Institute of Management Consultants USA