You ask: Is it bad form to call "ready to depart" when you know you won't be cleared yet? Speaking as a controller, my answer is emphatically:
Yes, this is bad form. Do not do it.
When you call ready for departure (and please do say "departure" rather than "takeoff" in all radio calls except reading back "cleared for takeoff" or "cancel takeoff clearance"), I have five options:
- Issue a takeoff clearance.
- Issue a line-up-and-wait instruction, if LUAW is available to me.
- Issue a hold-short instruction.
- Respond "Roger."
- Ignore you.
If I can't clear you for takeoff because there is an aircraft on short final, I cannot go with option #1 or #2. Then I must choose one of the other three. In order to prevent you from feeling left out, or concerned that I may not have heard you, I should ideally issue a hold-short instruction. This makes it very clear to all involved that you will remain short of the runway (which you were doing before). Unfortunately once I do issue the instruction I now have to make sure you properly read back the full instruction with your correct callsign, and if you don't I'm opening myself up to liability because now it isn't clear that you will remain short of the runway. The same problem exists if I simply respond "Roger," though less so.
So your calling up has not helped me in any way whatsoever. Instead I have to make more radio calls now in addition to the radio call I would have made anyway later once the runway was clear and I could issue the takeoff clearance.
Again, this is predicated on a situation where you see someone on short final and you know that you will not be issued any instruction other than "hold short." If you do a check and do not see anyone on short final, do call—there may be a fast jet on a three-mile final and I can't issue your takeoff, but if you didn't see them I can't hold you responsible for that. Or perhaps you notice them but you think they're still pretty far out and there's a chance I can get you going. Maybe you're correct, maybe not; if I can issue the clearance I will and if I need you to hold short I'll tell you.
But if you look out and see someone on a half-mile final, that is a good time to keep your PTT switch to yourself.
Once you have observed the traffic, I would say a good time to call ready is when they pass off your nose as you're holding short. If the controller is able to use LUAW they can issue that instruction immediately. If not, they can ignore you for the 20-30 seconds it takes to let the arriving aircraft touch down, slow down, and receive runway exit instructions—they're hoping that keeping the frequency busy will prevent you from calling again right away. Then they'll be able to issue the T/O clearance without having to go back-and-forth unnecessarily.
Finally, I would think that you would have completed all of your runup and associated checklists before lining up on the runway. LUAW is supposed to be used in preparation for an "imminent departure" which (our guidance says) means within 90 seconds of you taking the runway—and could be as soon as a second or two after receiving the LUAW instruction, even before you actually reach the runway pavement. I don't know what the standard procedure is at your airport, but if someone was holding in position and called me to say they were ready for departure I would be very confused, because I would have thought that was a given.