Solo flight and commercial ratings progress

Solo time is pretty straightforward, right?  If you're the only one in the airplane, then it's solo.  Anyone else aboard, then it isn't (and shouldn't be logged as such).

But as you advance in your ratings from private to instrument to commercial, you start to fly more sophisticated and more expensive aircraft.  Building solo time can be difficult due to insurance requirements around minimum experience, or due to the lack of a appropriate ratings (e.g., working on a multi-engine rating after earning commercial in a single-engine airplane).  If a student simply doesn't have the requisite level of experience, then they can't rent from an FBO, placing a burden on getting solo experience.

For this reason, the FAA has created a substitute for solo in the requirements for flight experience for a commercial rating (61.129).  The wording is that you must either be solo or "performing the duties of pilot in command...with an authorized instructor on board."

To indicate this sort of a flight on MyFlightbook, you should log the flight and attach the "Duties of PIC" (DPIC) property and the "Instructor on Board" (IOB) to the flight.  (I'll refer to this below as "DPIC/IOB" time)

There is often confusion about this, and I have on more than a few occasions received an email complaining that MyFlightbook is not crediting all of the time it should be crediting, or is missing their long cross-country flight.

The idea is that this is supposed to emulate solo flight, so the instructor is there not to teach you, but rather to satisfy insurance restrictions.  You're doing all of the work as if you were in fact in solo flight.  The instructor, therefore, should essentially be sitting on their hands for the flight.  The mistake people typically make here is that they read "instructor on board" as meaning "I can log dual." 

Unfortunately, this is a misreading of the regulation.  The normative legal interpretation from the FAA can be found in the 2014 FAA "Kuhn" interpretation (or Google the phrase "FAA duties of PIC Kuhn"): you cannot simultaneously receive instruction and count flight towards 61.129(a/b)(4).

The money quote from the Kuhn letter is "Because this flight time is a substitute for solo flight time, the pilot is not receiving instruction and therefore cannot log this time as dual instruction received."

For this reason, MyFlightbook subtracts out any dual instruction that is logged from the amount of  DPIC/IOB time to determine how much time can credit towards 61.129's requirements.

Note too, that since this is a substitute for solo flight, you must not log it as solo either (after all, you're not alone in the aircraft).

MyFlightbook computes a flight's contributions towards solo requirements as follows:

• Compute an effective DPIC/IOB time as
  

  MIN(DPIC/IOB, Total Time - Dual Received)

  So, for example:
• A flight with 2 hours of DPIC/IOB, 2 hours of total time, and 2 hours of Dual yields

  MIN(2, 2 - 2) = MIN(2,0) = 0.

• A flight with 2 hours of DPIC/IOB, 2 hours of total time, but only one hour of instruction, yields

  MIN(2, 2 - 1) = MIN(2,1) = 1 hour.

  I.e., I'm removing the hour of instruction per Kuhn, but the other hour can count as DPIC/IOB.
• A 2 hour flight with 60 minutes of instruction and 90 minutes DPIC/IOB yields

  MIN(1.5, 2 - 1) = MIN(1.5, 1) = 1 hour.

  I.e., the flight is ambiguous, but at most one hour of this flight could possibly contribute time here, so I'll give you the hour.
• Compute an effective solo time of

  MAX(explicitly logged solo time, effective DPIC/IOB)

  This value is what is used for total solo time requirements.  Note that this also implicitly corrects for accidental logging of both DPIC/IOB time and solo time.  I suppose it's possible that you have a flight where you fly with an IOB for a while, then the instructor gets out and you fly solo, but I also suspect that's pretty rare, since usually the whole reason you had to fly with an instructor on board is precisely because you can't fly solo.
• Compute an effective night VFR solo time of

  MIN(effective solo time computed above, Night - IMC)

  (Note that this could be a bit conservative: e.g., if you had a 2 hour solo flight where the first hour was in day IMC and then it cleared up and the 2nd hour was at night in VMC, then the flight won't credit.  But that's a bit of a corner case and it means that the error is conservative - better to understate than overstate experience!)  Anyhow, this value is what is used for any night solo time requirements.
• For any activities such as takeoffs/landings or a long cross country that must have been performed while solo, add them to your experience if the effective solo time is greater than 0.
• Finally, the various 61.129(4) requirements computed using both actual solo time and the effective substituted solo time.  After all of your flights are examined and the experience is summed up, MyFlightbook will report your progress using either exclusively actual solo time, or exclusively DPIC/IOB time (whichever has more time recorded), but not both.  This is because you cannot mix-and-match: all of the solo time in 61.129 MUST be actual solo time or it must all be DPIC/IOB time (see the FAA's 2016 "Grannis" interpretation for details). 

All of the above, of course, are in the context of 61.129, and thus only apply in determining your progress towards meeting the experience requirements therein.  MyFlightbook's 8710 form, on the other hand, is a summary of experience without the context of any particular rating.  As such, the 8710 form does not include the DPIC/IOB substitution, it only looks at solo time that is recorded as such.

So by all means, go out and build your "solo" time with an instructor on board.  But don't log it as instruction received.