Investigation into the Hop Bombardier CRJ1000 low-approach incident at Nantes in October 2021 has exposed vulnerabilities in crucial pressure-setting procedures, and potential difficulties in detecting errors.

The CRJ1000 flew the approach more than 500ft below the correct glidepath, a situation only discovered when air traffic control received a minimum safe altitude alarm.

French investigation authority BEA says that, after the crew incorrectly set the QNH pressure reference, the final descent path checks for the RNP Baro-VNAV approach did not enable the pilots to detect an altimeter-setting error.

BEA says this limitation is “inherent” to such approaches and a “well-known threat”, of which the operator’s pilots are “regularly reminded” during training.

Although cross-checking QNH or altitude values can pick up instrument or display errors, BEA says procedures such as distance-altitude checks during non-precision or Baro-VNAV approaches do not detect whether the QNH has been correctly set in the first place.

“A review of the procedures of different aircraft types showed that the validation of the QNH value by cross-checking it against another source of information is not described in detail in manufacturers’ procedures,” it states, although it acknowledges that such verification is not directly linked to aircraft operation.

aircraft on approach-c-Shutterstock

Source: Shutterstock

On certain types of approach, distance-altitude checks cannot pick up altimeter errors

It says there are “diverse” reasons for crews’ failing to carry out validation of the QNH, including training shortcomings or incomplete application of the procedure.

BEA says the radio altimeter was the only instrument which could have allowed the CRJ1000 pilots to detect the error, because it would have shown abnormally low readings.

But this check was “not clearly defined” by standard operating procedures in effect. Nor did the altitude displayed to the air traffic controller easily allow determination that the aircraft was not flying at the correct height.

BEA also recently investigated a serious mis-set altimeter event, involving an Airbus A320 on approach to Paris Charles de Gaulle, and the European Union Aviation Safety Agency has newly issued a safety bulletin after becoming concerned over a number of similar occurrences.

While EASA says that procedures relying on barometric altimetry have “considerably improved safety” by offering vertical guidance to runways, incorrect pressure settings can “severely affect” safety margins.

Setting the correct barometric pressure involves “several steps” that “may be subject to errors”, it adds, including the crew’s changing the reference after passing the transition altitude.

EASA believes a formal safety directive is not necessary, but it has made several recommendations to mitigate risks.

These include operators’ developing procedures to support pilots in checking QNH or QFE pressure references, ensuring that terrain-warning systems have up-to-date software and databases, and investigating methods to identify incorrect settings using flight-data monitoring programmes.

EASA suggests air traffic control services should consider introducing procedures to provide crews with QNH or QFE information when clearing aircraft for approach, or during first contact with the tower. It also says Mode-S downlink could help with timely identification of aircraft operating with incorrect settings.