Wind Correction (WIND CORR) Function - CDU — Flaps 2 Approach

OEM 737 CDU showing WIND CORR display in Approach Ref page

Wind Correction (WIND CORR)

The approach page in the CDU has a field named WIND CORR (Wind Correction Field or WCF).

WIND CORR can be used by a flight crew to alter the Vref + speed (speed additive) that is used by the autothrottle during the final approach. This is to take into account wind gusts and headwind that is greater than 5 knots.

Changing the Wind Correction to match increased headwind and gusts increases the safety margin that the autothrottle operates, and ensures that the autothrottle command a speed is not at Vref.

WIND CORR Explained

The algorithm of the autothottle includes a component that includes a speed additive. The speed additive is 1.23 times greater than the stall speed of the aircraft (at whatever flap setting). When the autothrottle is engaged, the speed additive is automatically added to Vref. This provides a safety buffer to ensure that the autothrottle does not command a speed equal to or lower than Vref. This added speed is usually 'bled off' during the flare ensuring landing is at Vref.

Although the autothrottle algorithm is a sophisticated piece of software, there is a time lag between when the sensors register a change in airspeed to when the physcial engines increase or decrease their spool (power). By having a speed additive (based on headwind and gust component) the speed of the aircraft (as commanded by the autothrottle) should not fall below Vref.

A Vref+ speed higher than +5 can be inputted when gusty or headwind conditions are above what are considered normal. By increasing the additive speed (+xx), the speed commanded by the autothrottle will not degrade to a speed lower than that inputted.

The default display is +5 knots. Changing this figure will alter how the algorithm calculates the command speed for the autothrottle; any change will be reflected in the LEGS page, however not in the APPROACH REF page.

The data entered into the Wind Correction field will only be used by the Flight Management System (FMS) when the aircraft is following an RNAV approach, or when using VNAV to fly an approach that has been manually constructed in the CDU. This is because these approach modes use the data from the FMS to fly the approach (as opposed to an ILS or other mode that doesn't use the FMS data).

If hand flying the aircraft, or executing another approach type, Wind Correction is advisory (you will need to add the speed additive (Vref+ xx knots) by mental mathematics).

Important Points:

Wind Correction is automatically added to Vref when flying an RNAV approach, or when using VNAV to fly an approach that has been manually constructed in the CDU.
Wind Correction is advisory for all other approach types or when manually flying an approach; +xx knots must be added to Vref by mental mathematics.

How To Use WIND CORR

The WIND CORR feature is straightforward to use.

Virtual CDU (ProSim737) showing the difference in landing speed with a Vref between a +5 and +13 Knot (Wind Correction) change. Vref altered from 152 knots to 160 knots

Navigate to the approach page in the CDU (press INIT REF key to open the Approach Reference page). Then double press the key adjacent to the required flaps for approach (for example, flaps 30). Double selecting the key causes the flap/speed setting to be automatically populated to the FLAP/SPD line.

Type the desired additive into the scratch pad of the CDU and up-select to the WIND CORR line. The revised speed will change the original Vref speed and take the headwind component into account. If you navigate to the LEGS page in the CDU, you will observe the change.

If the headwind is greater than 5 knots, then WIND CORR can be used to increase the additive from the default +5 knots to anything up to but not exceeding 20 knots.

It’s important to understand that the figure generated in the CDU is the Vref speed. This is the speed that the aircraft should be at when crossing the runway threshold or at a altitude of approximately 50 feet.

To this speed you must add the appropriate wind correction - either by mental mathematics or by using WIND CORR (if flying an FMS generated approach).

Boeing state that the +XX knots should be bled off during the flare procedure ensuring that touchdown speed is at Vref, however this rarely occurs in real life.

Recall from above, that any change using the Wind Correction field will have no bearing on calculations, unless the aircraft is being flown in RNAV / VNAV, or the approach has been manually constructed in the CDU.

For a full review on how to calculate wind speed, refer to this article: Crosswind landing Techniques - Calculations. A prompt sheet is displayed for quick reference.

Important Variables - Aircraft Weight and Fuel Burn

To obtain the most accurate Vref for landing, the weight of the aircraft must be known minus the fuel that has been consumed during the flight.

Fortunately, the Flight Management System updates this information in real-time and provides access to the information in the CDU. It's important that if an approach is lengthy (time consuming) and/or involves holds, the Vref data displayed on the CDU will not be up-to-date (assuming you calculated this at time of descent); the FLAPS/Vref display will show a different speed to that displayed in the FLAP/SPD display. To update this data, double press the key adjacent to the flaps/speed required and the information will update to the new speed.

How To Manually Calculate Fuel Burn

If wishing to manually calculate the final approach speed well before the approach commences, then it's necessary to manually calculate the fuel burn of the aircraft. Open the PROGRESS PAGE on the CDU and take note of the arrival fuel. Subtract this value from how much fuel you have in the tanks - this is the fuel burn (assuming all variables are constant).

Interestingly, the difference that fuel burn and aircraft weight can play in the final Vref speed can be substantial (assuming all variables, except fuel, are equal). To demonstrate:

Aircraft weight at 74.5 tonnes with fuel tanks 100% full – flaps/Vref 30/158.
Aircraft weight at 60.0 tonnes with fuel tanks 25% full – flaps/Vref 30/142.

Important Points:

During the approach, V speeds are important to maintain. A commanded speed that is below optimal can be dangerous, especially if the crew needs to conduct a go-around, or if winds suddenly increase or decrease. An increase or decrease in wind may cause pitch coupling.
If executing an RNAV Approach or using VNAV, it's important to update the WIND CORR field to the correct headwind speed based on wind conditions. This is because an RNAV approach and VNAV use the data from the Flight Management System (to which Wind Correction is added).
If an approach is lengthy (for example, during a STAR or when requested to hold), the Vref speed will need to be updated to take into account the fuel that has been used by the aircraft during the holding time.
Changing the WIND CORR speed in the CDU, does not alter the Vref speed displayed on the Primary Flight Display (PFD). Nor is the APPROACH REF page on the CDU updated. The change is only reflected in the LEGS page.
Boeing state that the speed additive should be 'bled off' during the flare so that the actual landing speed is Vref.

Autoland

Autolands are rarely done in the Boeing 737, however, if executing an autoland, the WIND CORR field is left as +5 knots (default). The autoland and autothrottle logic will command the correct approach and landing speed.

Simulated in Avionics Suite

WIND CORR may or may not be functional in the avionics software you use. Wind Correction is functional in the ProSim737 avionics suite.

Additonal Information

A very good video that discusses this in detail can be viewed at FlightDeck2Sim.

Acronyms

CDU – Control Display Unit
FMC – Flight Management Computer
FMS – Flight Management System (comprising the FMC and CDU)
Vref - The final approach speed is based on the reference landing speed
Vapp – Vapp is your approach speed, and is adjusted for any wind component you might have. You drop from Vapp to Vref usually by just going idle at a certain point in the flare