The Boeing 737 was on a scheduled service from Adelaide to Melbourne. The crew had flown a standard arrival route to a locator/distance measuring equipment approach to runway 27. The weather at the time was overcast with the cloud base at about 2,000 ft with drizzle. Extensive airport works were being conducted on and near runway 27. When the aircraft encountered visual conditions, the pilots found the aircraft high on the approach and attempted to regain the glide path by increasing the aircraft's rate of descent. As the aircraft approached 500 ft above ground level, the rate of descent was assessed as too high and the first officer called for a missed approach to be conducted, which was carried out by the pilot in command.
The aerodrome controller (ADC) instructed the pilot to maintain runway heading, to maintain 3,000 ft, and to call the departure controller radio frequency. Approximately 1 minute later, the departure controller advised the ADC that no radio contact had been made with the aircraft. Subsequent attempts to contact the pilot by radio on several other frequencies were unsuccessful. The departure controller also noted that the aircraft had climbed to 3,400 ft. After approximately a minute, the pilot re-established radio contact with the ADC and advised that he was unable to contact the departure controller. The ADC instructed the pilot to climb to 5,000 ft and to attempt to call the departure controller. The aircraft was observed to climb to 5,400 ft and subsequently the pilot established radio contact with the departure controller.
The pilots thought that the reason why they had been high on the approach was because they incorrectly transcribed the information from the operator's internal notice to airmen (INTAM) regarding amended procedures due to the aerodrome works. The departure controller requested the pilots to confirm the aircraft's altitude and it was during this check that the pilots realised that the barometric settings on the altimeters had not been set to the airfield QNH of 1028 hectopascals (hPa) but rather had been left on 1013 hPa; the setting required for flight above the transition altitude (10,000 ft.) As a result, the aircraft altimeters had under-read by about 450 ft. Once the pilots had corrected the error, the subsequent approach was conducted without further incident.
QNH is the mean sea level pressure derived from the barometric pressure at the station location. The local QNH at an airport is normally derived from an actual pressure reading. Australian aviation regulations require that, when an accurate QNH is set on the pressure-setting subscale of an altimeter planned for use under the Instrument Flight Rules, the altimeter(s) should read the nominated elevation to within 60 ft. QNH should be set on the altimeter pressure-setting subscale of all aircraft cruising in the altimeter setting region, which extends from the earth's surface to the transition altitude of 10,000 ft in Australia. QNE is the standard pressure altimeter setting of 1013.2 hPa that is set for flight above the transition altitude.
The works and consequent limitations on the use of the runway were detailed in a notice to airmen (NOTAM) and in an Aeronautical Information Publication Supplement (AIP/SUP). Some of the restrictions and amendments to the runway 27 usage included a displaced threshold, the installation of a precision approach path indicator (PAPI) vice the normal T-VASIS (which was unavailable) and non-availability of the high intensity approach lighting.
The first officer's experience was primarily on the 737-400 and the pilot in command had primarily flown the newer 737-800, which was equipped with significantly more integrated and up-to-date cockpit displays compared with the 737-400. One of the altitude indication features available on the 737-800 primary flight display (PFD) automatically highlights, in boxed amber, the barometric setting if the STD (Standard) QNE, rather than the local QNH is set and the aircraft descends through the transition flight level. The electronic altimeter is connected to the aircraft's flight management computer (FMC) and therefore can register if the subscale has been changed or not by reconciling the altitude to the database transition and whether or not STD is still set. The 737-400 cockpit displays did not have a similar indication because the altimeters are not connected to the FMC database that includes the aerodrome transition level/altitude information.
Both pilots were on the third consecutive day that required a 0600 departure. They both reported retiring the previous evening between 2000 and 2100. The first officer reported that he had some preoccupation with health issues involving his child. The pilot in command reported that on a later sector that day, he began to feel unwell, experiencing flu-like symptoms.
The instrument approach conducted by the crew was an operator-modified version of the published procedure. The amended approach was issued via a company INTAM that raised the published minimum DME altitudes by 73 ft. Because the modified approach was issued via text rather than a chart, the crew was required to transcribe the changes from the INTAM to their own in-flight briefing notes and they reported that they had taken some effort to ensure that they had transcribed the amendments correctly.
The operator had a sterile cockpit policy that applied from when the fasten seat belts sign was illuminated to when the landing gear was lowered. During this period, the cabin crew was not to contact the technical crew on the flight deck unless an urgent safety-related message needed to be passed. The fasten seat belts sign during this approach was illuminated at 20,000 ft.
The operator's pilots recall the checklist by referring to information listed on the yoke of the aircraft controls. A sliding marker was used to indicate where a checklist procedure was suspended to assist the pilots to regain the place in the checklist. In this case, the descent approach checklist included:
- Anti-Ice ON/OFF
- Air Cond & Press SET
- Altimeters & Instruments SET & X-CHECKED
- N1 & IAS Bugs CHECKED & SET
The operator's flight crew training manual required the descent approach checklist to be initiated during the descent and completed passing 10,000 ft. In practice, to enable a crosscheck of the altimeter settings, the pilots were required to stop the checklist at `Altimeters & Instruments' until the aircraft had descended below the transition altitude.
The pilots reported that after sterile cockpit procedures had been invoked and while they were conducting the descent approach checklist, a cabin attendant mistakenly contacted them on the intercom. The barometric subscales on the altimeters were not adjusted after they had recommenced the checklist following descent through the transition altitude.
A later opportunity to correct the missed check was lost when the pilots reported that they had crosschecked the altimeter settings and indications during the approach but they did not notice that the QNH had not been set. When the aircraft became visual at about 5 NM on final approach, the PAPI indication was four whites, indicating that the aircraft was high on glidepath. On seeing the airport, the pilots momentarily accepted the PAPI glidepath indication as being normal, as it was what they would have expected to see for an `on-glidepath' indication when using the T-VASIS.
Although they quickly realised their misinterpretation of the PAPI indication and the pilot increased the rate-of-descent to correct the aircraft's approach profile, the crew were unable to regain the normal approach as they approached 500 ft height above touchdown (HAT). Consequently, they conducted a missed approach as prescribed in the operator's flight administration manual. The manual advised pilots that an approach should be stable by 1,000 ft HAT and if the approach was not stable by 500 ft HAT, then a missed approach, or go-around, was to be conducted. The operator also stated in the manual that, `Flight Crew are encouraged to perform a Missed Approach whenever any doubt exists to the safe continuation of an approach and landing'.
The pilots reported that when pre-setting the frequency it was possible to dial the frequency on the radio control too quickly and the selection could overshoot by 0.25 megahertz. They reported that after they selected what they thought was the departure controller's frequency, the frequency channel appeared too quiet so they returned the selected frequency to the ADC frequency where they regained communications.
The operator reported that the general management processes taught to the company pilots were based on prioritising response and sequence management. This was associated with almost all of the training conducted by the operator, particularly during simulator training.
The non-selection of the QNH on the altimeter subscale and the incorrect selection of the radio frequency probably resulted from a combination of a number of factors. These included the:
- preoccupation of the first officer,
- possible slightly degraded performance by the pilot in command due to impending illness,
- interrupted checklist procedures, and
- high workload being experienced by the crew as a result of the meteorological conditions, works in progress and changed conditions at the airfield, and the amended approach requirements.
It could not be established whether the pilot in command's experience with the newer generation cockpit displays contributed to him missing the incorrect altimeter subscale setting. However, if the aircraft had been equipped with a similar display then the missed changeover from QNE to QNH may not have occurred.
Organisations require multiple defences to prevent hazards, such as human error, from becoming accidents. These defences may include procedures, training and equipment design. In this case, although procedures failed to stop the aircraft from descending below 10,000 ft with the incorrect altimeter subscale, other procedures such as the mandatory missed approach and the culture encouraging missed approaches prevented the error from compounding.
Local safety action
The operator has advised that its fleet type operating committee will review the approach checklist procedure, but that any proposed changes will require negotiation with the aircraft manufacturer because the operator's policy is to maintain its checklists consistent with the manufacturer's.
The operator is also examining using this event as a training discussion item for fleets with analog altimeters and will publish an account in its in-house safety magazine or operations newsletter.
|Date:||03 May 2003||Investigation status:||Completed|
|Time:||0744 hours EST|
|State:||Victoria||Occurrence type:||Unstable approach|
|Release date:||15 October 2003||Occurrence class:||Operational|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||The Boeing Company|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Nil|
|Departure point||Adelaide, SA|