Narrative:

Two rv aircraft were cleared as a flight of two for taxi to runway 20 at ksaf. I was the wingman; in the second aircraft (the rv-7). Although the weather was quite good directly over the airport; there was an 800 foot broken ceiling elsewhere in the class D that made the field IFR. During taxi out; ground control acknowledged a request from the lead aircraft for a special VFR clearance to depart the class D airspace. After engine runup; we switched to tower frequency and moved to the hold short line for runway 20. I heard nothing on the tower frequency; and began trouble-shooting to determine why not. I got it figured out (loose cable plug); verified good reception by listening to ATIS for one second; and went back to tower frequency; just as I observed the lead airplane pulling out onto the active runway. Because we were a flight of two; and my radio reception was good again; I followed; on his right wing. He accelerated down the runway; appearing to be making a takeoff; and I followed; as usual. (We do this frequently together.) he had gotten a little too far ahead of me on the runway for good formation work; perhaps 50 yards or so; so I increased throttle enough to start closing the gap. Then he slowed down; causing my closure rate on him to become excessive. I was comfortably off to his right; and had just barely become airborne; so I side-stepped further to the right and continued flying straight ahead; passing him. I was looking to my left for him; but could not see him. At this point; I assumed that he had aborted. I climbed straight out to about 700' AGL; then turned downwind and called the tower for landing; reporting that I had a business jet in sight; on short final for runway 20. I landed and returned to the hangar without further incident. The visibility and cloud clearance were very good for me during all of this; and I did not come close to any other aircraft. It turned out that while I was working on my radio reception issue; the flight leader had told the tower that he elected to not wait for the special VFR clearance; and wanted to return to the hangar. Tower told him to switch to ground for taxi clearance. Ground control then cleared the flight of two to utilize runway 20 for taxi southward to the midfield; then exit on taxiway golf. That was apparently said just as I came back 'online'; but on tower frequency; so I didn't hear it. The lead aircraft accelerated his aircraft to a very fast taxi; to reduce time on the active runway - that looked just like commencing takeoff; to me. I got airborne; he didn't; no loss of separation was involved that I could see; but I certainly took off without a clearance. I should not have followed the lead plane out onto the runway; since I never actually heard a clearance for the flight to do any particular thing. Not crossing the hold short line would have stopped this incident before it really developed.

Related reports:

Google
 

Original NASA ASRS Text

Title: RV-7 Pilot reported taking off on a formation flight when they had only been cleared to taxi off the runway due to a momentary loss of communication.

Narrative: Two RV aircraft were cleared as a flight of two for taxi to Runway 20 at KSAF. I was the wingman; in the second aircraft (the RV-7). Although the weather was quite good directly over the airport; there was an 800 foot broken ceiling elsewhere in the class D that made the field IFR. During taxi out; ground control acknowledged a request from the lead aircraft for a special VFR clearance to depart the class D airspace. After engine runup; we switched to tower frequency and moved to the hold short line for Runway 20. I heard nothing on the tower frequency; and began trouble-shooting to determine why not. I got it figured out (loose cable plug); verified good reception by listening to ATIS for one second; and went back to tower frequency; just as I observed the lead airplane pulling out onto the active runway. Because we were a flight of two; and my radio reception was good again; I followed; on his right wing. He accelerated down the runway; appearing to be making a takeoff; and I followed; as usual. (We do this frequently together.) He had gotten a little too far ahead of me on the runway for good formation work; perhaps 50 yards or so; so I increased throttle enough to start closing the gap. Then he slowed down; causing my closure rate on him to become excessive. I was comfortably off to his right; and had just barely become airborne; so I side-stepped further to the right and continued flying straight ahead; passing him. I was looking to my left for him; but could not see him. At this point; I assumed that he had aborted. I climbed straight out to about 700' AGL; then turned downwind and called the tower for landing; reporting that I had a business jet in sight; on short final for Runway 20. I landed and returned to the hangar without further incident. The visibility and cloud clearance were very good for me during all of this; and I did not come close to any other aircraft. It turned out that while I was working on my radio reception issue; the flight leader had told the tower that he elected to not wait for the special VFR clearance; and wanted to return to the hangar. Tower told him to switch to ground for taxi clearance. Ground control then cleared the flight of two to utilize Runway 20 for taxi southward to the midfield; then exit on Taxiway Golf. That was apparently said just as I came back 'online'; but on tower frequency; so I didn't hear it. The lead aircraft accelerated his aircraft to a very fast taxi; to reduce time on the active runway - that looked just like commencing takeoff; to me. I got airborne; he didn't; no loss of separation was involved that I could see; but I certainly took off without a clearance. I should not have followed the lead plane out onto the runway; since I never actually heard a clearance for the flight to do any particular thing. Not crossing the hold short line would have stopped this incident before it really developed.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.