Sir - I read the letter from Jack Karran about the Lima Boeing 757 accident (Flight International, 5-11 February, P41).
It should be noted that his suggestion that there should be the ability to test pitot/static "air-pressure sensors" would be adequate only if it were to be a full system test - not easy, as anyone who has had to conduct "huff-and-puff" testing on a pitot/static system will testify. Such testing requires disturbance of the system by the installation of adaptors over pitot tubes and static ports, a possible source of blocked ports if the adaptors are not removed before flight.
A sensor test would only confirm the co-operation of individual instruments and, as such, would not have identified the problem which triggered the confusion leading to the Lima 757 crash. Such a test would also probably require some form of disturbance of the system - for example, the isolation of the sensor from the system, to prevent back-leakage out of the ports. This, again, provides a potential source of errors.
The aircraft take-off run and initial climb-out, however, provide an excellent chance for the crew to check the operation of the pitot/static system. I have always called: "ASI [airspeed indicator] indicating" when accelerating down the runway, and then checked the vertical-speed indicator (VSI) and altimeter for operation in the correct sense - to check that they give readings which equate, at least roughly, to the anticipated performance figures as the aircraft rotates into the initial stages of the climb.
Cross-checking the ASI, VSI and altimeters operating off different pitot/static circuits will clearly indicate whether a particular circuit is malfunctioning, or if all are affected.
Using this method, problems with the pitot system will result in an aborted take-off, and problems with the static system will result in a circuit to land, or a similar manoeuvre. The drawback is that these checks must be conducted at a busy phase.
C H MORSHEAD
Langport, Somerset, UK
Source: Flight International
