QNH and MCP BARO on Boeing: Descent and Approach Altimeter Setup
On Boeing aircraft, forgetting the destination QNH during descent is one of the fastest ways to destroy an otherwise perfect NG ROUTE plan. The MCP BARO rotary sets the primary altimeter reference for altitude callouts, minimums, and glideslope geometry. Leave STD set while descending into a 1018 hPa airport and you may intercept the beam physically low while indications look on-path — or VNAV may compute a descent profile incompatible with terrain and approach gates.
Transition level and when to change
Climbing, you set STD passing transition altitude. Descending, reverse the process at transition level or when ATC provides QNH — operator procedures vary by region. European arrivals often use transition level printed on charts; US procedures may use ATC clearance phraseology. NG ROUTE arrival METAR displays the current QNH numerically — set it on the MCP when passing transition level inbound, not at the outer marker.
Where to set baro on Boeing
- MCP BARO knobs — captain and first officer altimeters should match
- Standby altimeter — cross-check independently
- CDU approach reference — some models store landing elevation and baro for RA minimums and approach briefing pages
- EFIS minimums — DA/MDA displayed depends on correct baro reference
In PMDG 737, verify both sides and the standby; call “QNH set” as PM challenge on approach brief.
VNAV and baro interaction
With VNAV PATH engaged, the FMC computed descent assumes correct baro for altitude constraints. Wrong QNH can bust step-down fixes on STARs even before the ILS. If constraints are missed, do not dive to catch up — execute missed approach or request vectors. NG ROUTE shows terrain along route; baro errors add hidden margin risk in mountains.
ILS glideslope and altitude relationship
Glideslope is geometric relative to the runway; your altimeter is barometric. Correct QNH aligns indicated altitude with true height above threshold. A 7 hPa error can mean roughly 200 ft indication error — enough to flare high or low and to confuse autothrottle on final. If GS pointer looks wrong but wind is calm, check baro before blaming the localizer station.
Recommended sim brief flow
- From NG ROUTE: note destination QNH and runway in METAR card.
- At top of descent: set MCP altitude, brief transition level for arrival.
- Passing transition level: set QNH both sides, verify standby.
- Approach brief: DA/MDA with correct baro, approach flaps and speed.
- 500 ft stable: altimeters agree within tolerance.
Live weather pairing
MSFS and X-Plane live weather approximate METAR; MCP must still be set manually in most Boeing add-ons. Read SPECI updates if your session is long — pressure changes slowly but matters near sea level in low-pressure systems.
Correct QNH is the bridge between NG ROUTE weather data and trustworthy Boeing approach indications. Treat BARO setup as a go/no-go item, not a afterthought during flap changes.
Procedures vary by operator and avionics suite. Real-world flight uses only approved charts and SOPs.