PERF APPR: REFERENCES, MINIMA AND APPROACH SPEED
PERF APPR gives the approach its working references: destination pressure, temperature and wind, together with landing configuration and approach-speed information. QNH is the local pressure setting used to indicate altitude with respect to mean sea level. STD is the standard-pressure reference used for flight levels. On arrival, the transition level tells us where the descent changes from flight levels back to altitudes; use the current arrival information and clearance. The MCDU's TRANS ALT field contains transition altitude, a different item. Typing QNH into the MCDU does not reach up and turn the altimeter selectors for us: set and crosscheck them at the applicable transition. [26, PDF pp. 199-209] [13, PDF pp. 149-158] [26, PDF pp. 205-207] [14, PDF pp. 1087,1089]
The weather names can look like another set of cockpit buttons. A METAR is an aerodrome weather observation, a TAF is an aerodrome forecast, and ATIS is the repeating airport-information broadcast. They help you build the arrival picture, but each item still needs its date, time and meaning checked. Wind direction also needs a north reference. True north is geographic north; magnetic north is the compass reference, with local magnetic variation between them. Check the reference supplied by the weather source and the reference requested by the receiving field. A field labelled MAG WIND is asking for magnetic direction. Apply the appropriate conversion once rather than assuming every report, broadcast and calculator already uses the same reference. [13, PDF pp. 145–146] [26, PDF pp. 205–207]
Minima define a decision point with a particular reference. A barometric decision altitude is read against the relevant pressure reference; a radio decision height uses height measured by the radio altimeter. Read the approach chart and use the correct type and value for the approach being flown. Similar-looking numbers can refer to different things. During preparation, establish the visual reference needed to continue and the response if it is unavailable. This gives the setting a practical meaning before the aircraft reaches it. The approach then has a decision we have already considered, rather than a new question arriving near the runway. [13, PDF pp. 149-158]
VAPP is the approach-speed target for the planned landing configuration and conditions. Check it against the accepted calculation and the aircraft's actual setup. A changed wind, runway, mass or configuration may affect the entries and the landing performance, so follow the change through the preparation. It is quite possible for the tablet, MCDU and physical aircraft to each contain a plausible value while referring to different versions of the plan. Bring them into agreement before the approach becomes busy. Knowing where the speed came from also makes it easier to recognise when an old entry has survived a later revision. [13, PDF pp. 149-158]
IN PRACTICE: Give PERF APPR useful information
Before you begin: The correct Heathrow runway and approach are in the flight plan, and current weather/landing-performance information is available.
1. Enter the pressure and temperature
Action. Open PERF APPR and enter the current destination QNH and temperature. Read each accepted value back and compare with the arrival information.
Check. The approach prediction uses the current pressure and temperature for the selected destination. [26, PDF pp. 205-207] [14, PDF pp. 1082-1084]
2. Use the wind reference the runway uses
Action. Enter the landing wind from the current tower/ATIS information, checking its reference. If starting from a METAR/TAF true-referenced wind, convert it to the runway reference before entry.
Check. The headwind component behind the approach-speed computation is not distorted by an unnoticed true/magnetic mismatch. [13, PDF pp. 149-158] [26, PDF pp. 205-207]
3. Keep TRANS ALT and transition level separate
Action. Check the PERF APPR TRANS ALT field against the destination transition altitude. Obtain the descent transition level from the applicable arrival information; it is not a separate TRANS ALT entry.
Check. The MCDU field and the later altimeter change each have their correct role. [26, PDF pp. 205-207] [14, PDF pp. 1087,1089]
4. Set the charted minima in the correct field
Action. Enter the charted minimum using its barometric-altitude or radio-height reference, as appropriate to the selected approach. Read the value and the reference back together.
Check. The approach minimum corresponds to the actual charted decision reference. A height has not been entered as though it were a pressure altitude. [26, PDF pp. 205-207] [13, PDF pp. 159-162]
5. Make configuration and VAPP agree
Action. Select the planned landing configuration, compare the resulting VAPP with the performance/briefing, and check the mass and wind information behind it. Revisit the result after a material runway, loading or weather change.
Check. The displayed approach speed belongs to the intended landing configuration and current aircraft/weather case. [13, PDF pp. 149-158] [26, PDF pp. 205-207]
6. Apply the local pressure reference at the right point
Action. When descending through the applicable transition level under an altitude clearance, set the current QNH on both sides and crosscheck the altimeters.
Check. The aircraft continues the arrival using the correct local altitude reference. Entering QNH on PERF APPR has not been mistaken for setting the altimeters. [14, PDF pp. 1087,1089]
Complete when: PERF APPR is complete, the minimum and wind references are understood, and both altimeters will be changed at the applicable transition point.