Flight Crew Air Transport Pilot (ATP)
Confusion; Situational Awareness
Aircraft Equipment Problem Critical; Deviation - Speed All Types
Flight Crew Returned To Departure Airport; Flight Crew Landed in Emergency Condition; General Maintenance Action
Aircraft
My partner and I picked up an airplane from maintenance that we had written up 2 days prior for an airspeed disagreement during takeoff that resulted in a RTO.  Maintenance had found debris and bugs in the pitot system. During the takeoff roll; we both were looking for any speed abnormality; but all instrument indications were normal; until climbing through approximately 500 FT MSL; when we got an amber boxed ALT indication on both PFD's. This was followed immediately by an amber boxed KIAS indication; with our instruments showing the left side nearly 40 kts higher than the right; and several hundred feet higher in altitude than the right side indication.  We decided right away to return to the field; requested this from ATC; and also assistance in verifying altitude and airspeed read outs from them.  Left side altitude seemed to match ATC's altitudes within a couple hundred feet at each altitude request; but the speed seemed faster than it should have been for the power settings used. At one point early on; as we still climbing to ATC assigned 3000'; our airspeed showed a 40+ kias difference; and altitude difference of nearly 1;000 FT.It was really difficult to establish whose a/s and altitude indications were correct; as we were trying to figure out where to level off; querying ATC on our altitude and airspeed (they gave us ground speed read outs that didn't match the airplane's ground speed readouts on either the pilot or copilot's side at all); and flying into IMC conditions.  In addition; the standby attitude indicator never consistently matched either side's instruments; another confusing element that went against expectations.  Once established in straight and level flight as we were vectored onto a downwind; after a minute or so; both airspeeds and altitudes gradually matched up.  With each subsequent descent; the right side instruments lagged grossly behind in indicating a descending altitude; with indicated speeds diverging as well. My copilot did an outstanding job of running all the necessary checklists. When we got to the part in the QRH where it directed us to select REV to revert to the 'correct' side's instruments; we opted NOT to do this; because we still could not determine that one side or the other was giving us the correct indications.  This judgment call was based on typical power settings not matching up with expected airspeeds; as well as ATC altitude read outs not matching either side's altitude indications with enough accuracy to give us confidence.  We briefed that we'd fly the approach (ILS as a back up to a visual) using known power settings for our weight and the outside temperature; and use the AOA as a reference all the way down; being ready to go missed immediately with any sign of a stall shaker or buffet. We expected the right side airspeed to show slower; as it had been throughout most of the flight; but during this approach; once fully configured and stabilized; the right side KIAS actually indicated about 15 knots FASTER than the left; and appeared to now be the more accurate speed to follow based on the power setting and AOA indication.  VERY con fusing; very mind meddling.  We mostly followed the AOA and landed without incident.I thought we handled this very well in terms of procedures; checklists; pace; communications; and utilizing resources. I will say that this incident defied all kinds of expectations in terms of the instrument errors we were seeing. In training; with a little time and thought; altimetry and airspeed errors can usually be worked out and the faulty side isolated (using REV to select the 'good' side).  This situation wasn't clear cut at all; and as a matter of fact; after landing; we realized that on the previous flight in which we'd written up the airspeed mis-compute; the higher airspeed side was the reverse of what it was for 80% of this flight. I did realize as I was writing the narrative above that we never actually briefed the ILS approach; which weended up flying until about 5 miles out. We'd seen the airport on downwind and were planning a visual; but lost sight of it as we were vectored over the water.  We could have briefed that a little better!
All instruments were normal throughout the takeoff roll. Shortly after rotation; I (as the non flying pilot sitting in the right seat) noticed the amber 'IAS' and 'ALT' warnings on both my PFD and the left side PFD. After comparing airspeed indicators and altimeters I noticed that our airspeeds were approximately 40 KTS different from each other and our Altimeters were approximately 700-1;000 feet different from each other. At that time we told ATC that we wanted to return and started asking them for Airspeed and Altitude readouts from their radar. All of our information from that point on was unreliable. ATC continued to give us airspeed and altitude readouts that were consistently 40 KTS different than all three of our indications and +- 800 feet different from all three of our altimeters. We ran the appropriate checklists and returned.We believe that the information from our instruments was most accurate on the left side during climb out; but after leveling off at approximately 3;000 feet and returning; we believe the right side became more accurate on approach and landing.Two days prior to this event we had this same aircraft. That day; we executed a high speed abort due to the same amber IAS warnings. Maintenance worked the discrepancy and returned it to service documenting that they 'Purged a large amount of bug and vegetation debris from the pilot side pitot system.'
HS-125 flight crew experienced pitot static system malfunctions shortly after takeoff and returned to the departure airport. None of the airspeed indicators appeared to be functioning normally and AOA was used for the approach.