Sooner or later after takeoff it will be critical.
Not necessarily. Personally, I consider taking off VFR to be fatally flawed unless ceiling and visibility are sufficient to allow me to remain in VFR conditions long enough to land at my airport of departure. The only exception, for me, would be if I were sure that I could remain in VFR conditions to another airport within my fuel range.
In the first place, you decided to depart with an airborne pickup because you had reason to believe there would not be a delay, some fools excepted. Otherwise, you'd have got it on the ground.
That is not the reason I have done it in most cases, and in the few times when it was, conditions were good enough so that it was apparent before takeoff that I would be able to get back on the ground if I was unable to get the airborne pickup. (There was one possible exception that I can think of, but I learned from the experience and raised my standards for what weather information I need to have before attempting it.)
Some examples of situations in which I have gone for an airborne pickup are as follows:
- The weather was VFR where I took off and IFR at my destination. I was departing in a piston single from TRK (in the Sierras) on a flight to the SFO area, and an IFR departure would have required far more climbing than was needed. I didn't request my IFR until somewhere around Sacramento.
- For a flight from South Lake Tahoe (TVL) when they still had a control tower, the winds were favoring a takeoff to the south, but among other issues, the Cutlass I was flying lacked the climb performance required for the ODP from that runway. The winds were too strong for a tailwind takeoff, so I took off VFR, made a downwind departure, and followed the localizer out over the lake, thus solving the climb gradient problem. While I was still in VFR conditions, I called for the pop-up clearance. The controller expressed reservations about the operation but didn't ask the terrain-avoidance question, so I volunteered the information, and she granted the clearance. If necessary, getting back to TVL in VFR conditions would not have been a problem.
- On a VFR departure from a Seattle area airport to the Portland area one winter (again in a Cutlass), I was planning to avoid potential icing by climbing in VFR conditions until I was above the reported tops to the south. When I got above 11,000, I discovered that the headwinds were so strong that I was making five knots ground speed according to the DME (!), so I went back down and requested an IFR clearance. This time, the controller did ask the question, and given my location and what I could see underneath the ceiling ahead, it was obvious that there would be nothing to hit towards Olympia (OLM), so I told him that I could do it by climbing direct Olympia. If I hadn't gotten the clearance, landing somewhere in VFR conditions would not have been a problem. (Fortunately, I didn't pick up any ice.)
- On a VFR flight up the Oregon coast under the ceiling, I came to a wall of clouds that went all the way to the surface. I circled in VFR conditions while I called for a pop-up clearance, and said that I could maintain terrain and obstruction clearance by climbing to the west. (Duh!) I don't remember if the question was asked or if I volunteered the information. (One of the nice things about the Oregon Coast is that there's an airport about every ten minutes, so turning around and landing would not have been a problem.)
- On a VFR flight from the SFO area to Bishop (BIH), I encountered lower than expected ceilings ahead, over the Sierras. I called for a pop-up clearance, and ended up having to fly a much longer distance, and at higher altitudes, than I had planned. (That was the flight that caused me to buy an oxygen tank.)
This is just a sampling, but in summary, the most common reasons I have taken off VFR and picked up IFR while airborne have been weather and/or performance issues, not because I thought it would save me time.
So, off you go in your biz jet climbing at three or four times the climb gradient assumed on a diverse departure (which is just 200'/nm). Next, ATC reneges on their promise of a timely pickup at pattern altitude, so you need to pull back the power, slow or stop your climb to avoid the ceiling and bide your time at LESS than the assumed gradient. So, now you're using up the surplus altitude you gained and moving downrange from the airport at four miles a minute without even a sectional chart. The pressure mounts as you're trying to decide how far you can go before needing to do a 180 in the increasingly worsening visibility under the ceiling.
I've never flown a biz jet and I probably never will, but for the reasons you mention, in addition to the larger turning radius of a faster airplane, in order to take off VFR in one I would require even better weather than for the piston singles that I normally fly. If there were not room to comfortably and safely circle in VFR conditions while waiting for my clearance, I would not take off VFR.
Finally, ATC has your clearance, but before they can release it they want to know "Can you maintain terrain clearance below MIA?"
"What? Of course not, that's why I need an IFR clearance--so I can get higher."
Then the back and forth conversation ensues where they try to explain what "the question" means. BTDT.
That reinforces what I said earlier about needing more pilot training on the implications of the question (and the answer).
I like the question because I think that making sure everyone is on the same page about who is responsible for what is good for safety.
