so, you overhead the airport, make a big teardrop turn to intercept the ILS, oh and lose3 10 or 20 thousand feet in the meantime at idle thrust. fuel efficent with minimum of navaids.
sevenstrokeroll, one of the reasons that the Jet Penetration was either canceled or modified was due to altimeter lag that the old style altimeters had, which were installed in the early jet aircraft.
During my father's career in the US Air Force he was assigned to the Accident Investigation Branch. There had been an F-86 that had crashed at Kirkland AFB located at Albuquerque, New Mexico. The F-86 had been cleared for the penetration and impacted the ground, on course but well below the altitude limit.
So my father flew a T-33 out to Kirkland. The weather was better than when the F-86 had crashed, but was still low enough for my father to use the penetration procedure. I have forgot the altitude he started the procedure, but it was about the same as the F-86 was at.
During these penetrations the Vertical speed indicator would be pegged at the bottom of the scale, not giving the pilot the actual rate of descent. My father came out of the bottom of the cloud deck three thousand feet lower than the altimeter was showing him to be at, heading directly at where the wreckage of the F-86 was. Even then he had pull a lot of Gs to keep from adding more aircraft parts to the wreck of the F-86.
He of course reported this and after a lot more research Jet Penetrations procedures were change and the altitude limit for the procedure were raised. At least until a new type of altimeter was made that could keep up with the rate of descent required for this procedure.