From personal experience (I didn’t stop); this topic, and my incident, raises some very interesting operational and human issues. The circumstances of my incident included corporate attitude, peer pressure, macho, and desire to help. However the primary issue was that the company procedure was ill-defined. After a full and frank debrief and self initiated investigation, the following was concluded:
The purpose of procedure - NFP calls 80kts - was twofold:
a) To crosscheck the ASI for errors. (N.B. PF assumed to check his ASI, but not written in the procedure).
b) To define the point beyond which an RTO is not required for a single system failure.
The investigation found that the crosscheck was a hang over from a previous operation with an older ASI sysyem and CAA inspector suggestion – it seemed to be a good thing to have. Whereas a risk assessment of the ASI / pitot static system (should be available from most manufacturers) suggested that there was a low probability of failure. (This was a modern aircraft, but with conventional instrument system).
During take off the PF would be able to check his ASI; although slightly later than 80 kts, thus at a higher speed, but training and experience enabled an accuracy check of +- 2 kts, enough for any gross error. However there was no procedure guidance as to what to do in the event of an error. The assumption was that it was a single system failure, and above 80 kts no RTO was required.
The technical debate re continuing the take off considered the issue of flight with an ASI error. Excluding the obvious gross error or total failure, the only way to determine the serviceable ASI was by comparison with the third / standby system – but there wasn’t one! The certification standard of the aircraft did not require it; later variants of the aircraft are fitted with ADCs / EFIS and have a standby ASI/Altimeter.
The certification basis of the aircraft protects the aircraft from an early rotation, limited elevator effectiveness (Vmu), and there was some protection for over-speed by the aircraft self-rotating; I also expect the end of the runway appearing would assist. For climb-out and subsequent flight the certification requires the aircraft to be in trim at V2+10; thus the PF, without over-reacting, could achieve a safe altitude to asses the situation. However nothing was found in the regulations as to how the crew would establish which ASI was correct; no doubt many assumptions based on airmanship and experience – noise, feel, trim, and power. An excellent simulator exercise for CRM and problem solving (MEL flight without the stand by ASI – the MEL assumes that the risk of ASI disagreement is low, but not impossible).
I could not find any certification requirements for an incapacitation check; this seems to be yet another of those items that could be introduced (operator or CAA inspector) because they seem to be a good thing to do i.e. without rationale. I suspect that the probability of PF being incapacitated is far less that a multiple systems failure that could have far more serious consequences.
The current SOP on an EFIS aircraft requires an 80kt call; this is a check that power is set and that thereafter an RTO is not required for a single system failure. ASIs are checked for gross error / failure. In the event of an ADC error the EFIS comparator gives a low-level ADC alert; if time permits the third ASI/Altimeter system is checked. The aircraft is flown to a safe altitude and the situation re-evaluated using the third ASI to resolve any ambiguity. The certification of a low-level alert suggests that the CAA condone continuing the takeoff; this may be due to the alert being an ADC comparator, the error could be in the ASI, VSI, or altimeter. Thus at 80 kts the Captain can still decide to abort, his call: if so this may be the safest option overall. Aircraft certification does not take into account, poor weather, inexperienced crew, complex ATC scenarios, high terrain, and all of those airframe systems that use ADC input (which we all know well, at least on examination day).
Epilogue:
Company procedures rethought, judged against the quantifiable risks for the aircraft type / operation, and now rewritten. Captains are in command; have a no blame / fair safety culture. Personal self-assessment / de-brief complete, studying for CRM instructor ticket. Beware of introducing procedures without a risk assessment and any well thought out rationale. Always ask the inspector ‘why’, and get a full justification for any request to introduce additional procedures (requirements etc); there are enough vital activities to do with more clutter.