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Background: The diversity of the Emirates network results in the requirement to conduct VNAV
approaches at numerous destinations. The following document has been developed in
conjunction with Boeing in order to add additional information to that already found in the FCOM
and FCTM.
VNAV: A Recap
VNAV operates in 6 phases of flight:
1.
2.
3.
4.
5.
6.
VNAV take off
VNAV climb
VNAV cruise
VNAV descent
VNAV on approach.
VNAV go around (a replication of climb and cruise)
We will concentrate on phase 4 and 5 of VNAV flight as they seem to create the most confusion.
We will also include a few reminders and tips for the first 3 phases of flight. Since the go around
is a replica of climb and cruise we will not discuss it in this document.
1) VNAV take off starts from initial application of thrust until CLB Thrust reduction. VNAV uses
baro altitude to compute the height of the airplane above the runway. This information is then
used as the reference ground elevation for computing the height of the airplane above the
runway when performing the VNAV take off steps [e.g. 400’THR REF and VNAV SPD, 1000’‐
acceleration]. The take off profile events are entered as heights not altitudes.
2) VNAV climb starts from CLB thrust reduction until VNAV captures FMC Top of Climb point
(T/C). During a VNAV climb, the FMA can only display THR REF for the thrust mode and VNAV
SPD for the pitch mode. During the climb, if VNAV ALT engages we know that the MCP is
“blocking “the FMC [i.e. the MCP altitude is lower than the altitude in the FMC; setting a higher
altitude in the MCP altitude window and pressing the altitude selector once will initiate further
climb in VNAV SPD. If ATC issues a higher cruise altitude than planned in the FMC, selecting the
new altitude in the MCP altitude window and pressing the altitude selector will cause the FMC
cruise altitude to automatically change to the new MCP altitude, the climb will
commence/continue in VNAV SPD. If ATC issues a final cruise level below the FMC planned
cruising level, you cannot lower the FMC altitude by pressing the altitude selector; instead you will
need to type the new lower altitude into the FMC to ensure the MCP and FMC altitudes are
matched. Caution is required with this action, as any altitude constraints above the new cruise
altitude will be automatically deleted.
Unfortunately, we are not always quick to pick up when VNAV path is not doing what we intend it
to do. Often we see the aircraft level off at an altitude lower than the MCP altitude with VNAV
PTH annunciated on the FMA. Since we are expecting VNAV PTH we are not surprised, but
VNAV is actually following an altitude constraint in the FMC [i.e. the MCP altitude is higher than
the FMC constraint which we have not removed by pressing the MCP altitude selector or typing in
a new altitude in the FMC — watch out for this. (E.g. 6000’ altitude constraints at several
waypoints in the DVR SID off RWY27 at LHR or when you set a higher cruise altitude in the MCP
altitude window without removing the FMC constraints). Also, whilst on climb be careful of dialling
in a higher altitude in the MCP window and then “just pressing” the altitude selector as habit. This
misunderstanding of the system could result in the unintended deletion of altitude constraints in
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the LEGS page. The only time the altitude selector should be pushed on climb is to either resume
a VNAV climb after maintaining an intermediate altitude, or to intentionally delete altitude
constraints on the LEGS page.
3) VNAV cruise begins when VNAV PTH engages at top of climb. With VNAV PTH engaged the
speed window may be opened without disengaging VNAV PTH [e.g. turbulence in cruise]. As we
will see later, this is an important point to remember. The only time we cannot open the speed
window and expect VNAV PTH to remain engaged is while tracking the descent path prior to the
VNAV ‘on approach’ phase.
4) VNAV descent begins at top-of-descent. Concurrently, the thrust mode changes from SPD to
IDLE to HOLD. During descent, the computed path has priority over the programmed speed. As
we know, changing environmental factors (e.g. wind) during the descent (compared to FMC
forecast conditions) sometimes means there are speed deviations from the VNAV PTH
programmed speed. So long as your speed remains within the defined parameters described in
FCOM 11‐31‐22, the FMC will ensure you remain on the descent profile and all speed and
altitude constraints will be met. In VNAV PTH, speed loss greater than the defined parameters
will cause the thrust to increase until the speed is restored to the FMC programmed descent
speed. [Thrust mode changes to SPD.] If the aircraft gets too high on profile after increasing
speed to its upper speed parameter, the FMC message “Drag required” will display. Without
intervention VNAV PTH will change to VNAV SPD. In this case, the speed target is automatically
set to 16 knots below Mmo/Vmo, which is typically higher than the planned descent speed, to
facilitate an airplane flight path that will converge to the VNAV descent path. However, use of
speed brakes will most likely be necessary to ensure that the descent path is intercepted. This is
important, because often pilots think that VNAV SPD will try to regain the profile; not so, it has
given up and you need to intervene to sort out the profile. If the pilot has not opened the speed
window, VNAV PTH will automatically reengage upon intercepting the profile.
Another technique for regaining the path from above [other than using speed brake] would be to
open the speed window to increase the speed. Whilst this is desirable from a passenger comfort
perspective, it prevents VNAV PTH from automatically re-engaging when reaching the computed
profile. Remember what we said earlier, in the VNAV descent phase you cannot have the speed
window open with VNAV PTH engaged. When the aircraft intercepts the required path, you will
have to close the speed window to get re-engage VNAV PTH. You can proactively help the FMC
maintain the programmed speed by applying a few simple techniques.
1. If your speed gets slow and the thrust mode is HOLD, you can manually add thrust to regain
speed.
2. If your speed gets too fast on descent, you can use the speed brake to maintain the
programmed speed.
3.
You can change the programmed speed by manually adjusting the FMC descent speed to
match the current aircraft IAS but this might consequently disengage VNAV PTH.
Remember, the FCTM 5.37 requires all intermediate descent altitudes constraints prior to the
FAF to be set on the MCP, until compliance is assured. If the crews are slow in setting the next
intermediate altitude, VNAV ALT will engage when approaching the current constraint. To
reengage VNAV PTH a lower altitude will need to be set on the MCP and the altitude selector
pressed once to initiate the descent. If the next constraint altitude is assured crews may set the
next lower constraint altitude, but care must be taken not to press the altitude selector when
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setting the new constraint altitude as this will result in the deletion of the next constraint. As with
the climb, “just pressing” the altitude selector as a habit, can result in the unintended deletion of
the altitude constraints. Again, you should know when it is required to press the altitude selector
and when it is not required.
5) VNAV on approach is the fifth VNAV phase and also the one that seems to cause the most
confusion. VNAV descent ends when VNAV transitions to on approach. FCOM 11.31.25 lists the
criteria that trigger the on approach phase but it is important to remember there are no additional
FMA annunciations.
Now remember in point 3 - VNAV cruise), we mentioned that opening the speed window on
descent in VNAV PTH will cause VNAV SPD to engage and VNAV will not capture the computed
path. During the VNAV on approach phase this is not the case. If VNAV PTH is engaged and the
on approach phase is active, VNAV PTH will remain engaged when the speed window is opened
and VNAV will capture the VNAV path pointer. Progress page 2 will also indicate the vertical
profile
Imagine we are descending in VNAV SPD (speed window open) and we call for Flaps 1 in
preparation for the approach. If VNAV on approach criteria are met [Flap and other things trigger
the “on approach logic”], VNAV mode automatically changes to VNAV PTH and the speed
window remains open. If you are below the path, the aircraft will level off until path interception, if
you are above the path, VNAV PTH will remain engaged and attempt to capture the path from
above. If it is unable to capture the path from above VNAV SPD will annunciate and the pilot will
need to manage the profile through an increased rate of descent until VNAV PTH can
automatically re engage. [The same as a normal descent mentioned in point 4 – VNAV descent]
Once VNAV on approach is engaged with VNAV PTH displayed, VNAV will maintain the
computed vertical profile regardless of IAS selector changes [FCOM 11.31.21]. The A/T SPD
mode controls the thrust to maintain the airspeed. So although we seldom see this, the VREF +5
kts speed on approach may not be maintained exactly.
Also, if ATC were to assign you an altitude of 2000’ whilst “on approach” and the aircraft was
maintaining 3000’, selecting 2000’ on the MCP and pressing the altitude selector will not initiate
an immediate descent. Instead VNAV will command level flight until intercepting the computed
descent profile. If an immediate descent is required (e.g. - ATC require a minimum of 500’/min),
then a means other than VNAV must be used to descend to the lower assigned altitude. The best
way to commence an immediate descent is to revert to a basic mode of V/S or FLCH SPD.
VNAV ALT Discussion:
VNAV ALT is a common pitch mode whilst maneuvering for a VNAV approach. This is particularly
evident when an aircraft has departed from an airfield and has decided to return to the airfield of
departure. In this case, the FMC cruise altitude has not been reached, the MCP altitude is set to a
lower altitude than the FMC altitude and VNAV ALT has engaged at the MCP altitude (see point 2
– VNAV climb).
There appears to be some confusion amongst pilots regarding the manipulation of the MCP/FMC
when VNAV ALT is the engaged pitch mode.
VNAV ALT is an acceptable mode when at or below the platform altitude and approaching the
Descent Point “D” for the approach.
Two conditions are required for VNAV PTH to automatically engage when VNAV ALT is the
active mode:
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1. The MCP ALT window must have a lower altitude than the current altitude, such as MDA.
This allows the aircraft to descend at the Descent Point (D).
2. The FMC must not be in the VNAV Cruise phase.
Note:
You can easily check the present FMC - VNAV phase by first selecting any FMC
button except VNAV. Thereafter, selecting the VNAV button will display the active
VNAV page! [I.e. if VNAV PG 2 displays, the aircraft is in the cruise phase, if
VNAV PG 3 displays, the aircraft is on descent phase].
So, in the absence of a cruise altitude in the FMC and VNAV descent being the active mode,
VNAV ALT will automatically change to VNAV PTH just prior or just after the Descent Point “D”,
no further pilot action is required. In some cases the VNAV deviation scale and pointer may not
be visible until shortly before the computed descent point.
Pilots may decide that they wish to engage VNAV PTH many miles prior to commencing the
approach. This makes us feel comfortable because of our perception that VNAV ALT is
undesirable and VNAV PTH is desirable! There are a variety of techniques that enable you to do
this however if not clearly understood, some of these techniques could have unintended and
detrimental consequences. The four main techniques for manually changing VNAV ALT to VNAV
PTH, in preparation for an approach will now be discussed:
1. Perhaps the simplest technique, and the one preferred by Boeing, is to select any other
MCP Pitch mode other than VNAV (ALT or VS 0/0) and then reengage VNAV. If an
approach has been selected in the FMC and the aircraft is maneuvering for an approach
with VNAV ALT engaged, selecting ALT HOLD will replace VNAV ALT with ALT on the
FMA, reselecting VNAV should cause VNAV PTH to engage. This technique will only
work if an approach has been selected in the FMC prior to the MCP pitch selections.
2. The second technique is to ensure that the FMC cruise altitude is changed to the current
cruise altitude and the altitude selector is pushed prior to entering an approach in the
FMC. Entering your current cruise altitude in the FMC and pressing the altitude selector
will cause VNAV PTH to engage. This technique should only be used if VNAV PTH is
engaged prior to selecting an approach in the FMC. We will discuss the possible
unintended consequences of engaging VNAV PTH after selecting an approach in the
FMC below in point 3.
3. The third, and fairly common technique for pilots, is to manually insert a cruise altitude in
the FMC cruise page prior to the approach but after selecting the intended approach in
the FMC. It is important that pilots are aware that significant problems can arise from this
sequence of event.
Let’s use two examples to explain:
i) You are maintaining 3000’; inbound to ZADAD [FAF] for the KJFK RWY 13L SAAAR
approach. You insert 3000’ in the FMC VNAV page 2/3 and execute it. You then
press the altitude selector once to engage VNAV PTH. This action allows you to
engage VNAV PTH which is desirable, but the unintended consequence is that the
hard altitude (big font) of 3000’ at ZADAD is deleted and replaced with a soft altitude
(small font) of 2950’ [This does not always happen, the altitude may remain the same
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but it will always change to small font] This action could violate the approach
validation criteria if the changed altitude is at or after the FAF. Remember, you are
not allowed to modify an FMC altitude at or beyond the FAF! [Refer FCI/FOMmodifications for VNAV approaches] however FMC speed modifications are
acceptable.
To resolve this undesired side effect, it is acceptable to re-insert 3000’ at ZADAD.
Unfortunately, this technique may not always work as the FMC may not accept an
altitude constraint that is equal or higher than the cruise altitude when it is the next
waypoint sequenced in the FMC.
Re-inserting an unintentionally modified altitude is not considered a violation of
SOP’s [no modification of altitudes at or after the FAF], but crews must be cautious of
being “heads down” during such a critical phase of flight.
Note: A quick and easy way to check if you have unintentionally modified any
altitudes on the approach is by checking the VNAV descent page, Line 1R. This
line displays the next waypoint constraint from the RTE LEGS page (see FCOM
11.43.4). When approaching the descent point of an approach, you would expect
to see either a bold magenta altitude or a bold magenta ‘At or above’ altitude. If
this is not case, there is a good chance you have unintentionally modified an
altitude constraint.
ii) Our second example deals with a scenario where the aircraft returns to land shortly
after take off and hence proceeds directly to the approach. This would potentially
result in the intermediate arrival constraints being bypassed which would also
simulate an ATC radar vector onto the approach. The LIDO chart is attached for your
reference: After passing 2000’ on departure climb from KSEA (Seattle) the crew is
asked to level off at 4000’ and to return for a LOC DME approach 34R [LIDO 7-60
KSEA]. The PF sets 4000’ in MCP (FL 350 is the FMC cruising altitude).
Approaching 4000’, VNAV ALT engages (see point 2 - VNAV climb).
The ILS 34R approach begins at KLEWS (12 ISEA) at 4000’ with the FAP at BUCKK
2200’ (6.2 ISEA). From BUCKK the FMC shows a GP angle of 2.99 degrees. There
is no FMC GP angle displayed from KLEWS to BUCK.
However, the LOC DME 34R approach begins after KLEWS (11.5 ISEA) at 4000’
with the FAP at BUCKK 2320’ (6.2 ISEA).
Crews should be aware that the charted ILS GP angle and the LOC GP angle may
sometimes differ (see LIDO chart) and in order to ensure the approach validates, the
correct approach must be selected in the FMC. If the pilot was to select the 34R ILS
approach in the FMC, the 34R LOC DME approach would not validate so a basic
vertical mode is required to fly the approach. This is because the 34R ILS requires a
GP of 2.75 degrees [FMC shows a GP angle of 2.75 degrees] but the 34R LOC DME
requires a GP of 3.00 degrees (>0.1 degrees). In this case, selecting the LOC 34R
approach in the FMC should ensure the approach validates. If the FMC does not
contain a separate LOC approach in the database, crews are permitted to select the
ILS approach from the FMC providing it meets the validation criteria. In this case,
crews will need to pay particular attention when validating the GP angle as there may
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be a significant difference between the LOC and ILS approach glidepath angles (see
next page). Also, consideration should be given to any step down fixes that may be
present on final. Some LOC-only procedures may not clear the step-downs when
the ILS G/S will.
After preparing for the approach the PF decides he would rather have VNAV PTH
engaged and instructs the PNF to enter 4000’ in the VNAV Cruise page and then
presses the altitude selector to engage VNAV PTH.
By entering a cruise altitude of 4000’ in the VNAV Cruise page, the 4000’
requirement (bold Font) at KLEWS is automatically deleted in the FMC and is
replaced with a computed crossing altitude (small Font) of 2820’, that’s nearly 1200’
lower than expected!
In this example there was no displayed FMC GP angle from KLEWS to BUCKK. This
meant that having got rid of the 4000’altitude at KLEWS, the FMC calculated a
shallow descent from 4000’ to the hard altitude at BUCKK 2320’.
To counter this dangerous consequence and to fly a rule compliant STAR you would
need to re-insert 4000’ at KLEWS; but as discussed, this action may not always be
possible.
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LOC
DME
GP
ILS GP
4. The fourth and final option available to crews is to enter a higher altitude in the FMC
VNAV cruise page than what is in the MCP altitude window. You can then press the
altitude selector to engage VNAV PTH. The benefit of this technique is that no altitude
constraints are deleted and VNAV PTH is engaged early. However, unless the pilot fully
understands what phase VNAV is operating in, this technique can cause significant
confusion. If the aircraft is too far from the descent point when the higher altitude is
entered, or the altitude entered in the FMC is not high enough above your current MCP
altitude, the FMC may automatically enter into the climb phase (as indicated by a green
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T/C on the ND). Pressing the altitude selector will not engage VNAV PTH. The aircraft will
have to pass the green T/D point and the pilot will need to then press the altitude selector
to initiate a descent. Pressing the altitude selector prior to the descent phase will have no
effect on changing the VNAV mode. To add more confusion to this technique, if the pilot
forgets to press the altitude selector (to engage VNAV PTH) when VNAV enters the
descent phase, VNAV ALT will remain the active mode at the descent point and the
aircraft will not descend as planned. The pilot must press the altitude selector to
commence the descent and approach.
So what have we leant?
1. When approaching the descent point “D” of an approach, VNAV ALT is an acceptable
mode provided VNAV is in the descent or approach phase and a lower altitude is set on
the altitude selector. VNAV ALT is not an undesirable mode if you understand it.
2. If you intend to engage VNAV PTH prior to its automatic engagement on an approach you
have four techniques available to you.
i) Select another pitch mode to remove VNAV ALT, and providing you have entered
an approach in the FMC, VNAV PTH will engage when VNAV is reselected.
ii) Match the FMC and MCP altitude prior to entering the approach. This will allow
the engagement of VNAV PTH without deleting any approach altitude restrictions.
iii) Insert your current altitude into the FMC cruise page to engage VNAV PTH;
however you may delete altitude constraints on the approach. Corrective actions
are required in such a case to manage the threats and to ensure a rule complaint
approach.
iv) Insert a higher altitude than your current altitude into the FMC, no altitude
constraints will be deleted but full awareness of the actual VNAV operating phase
is required to avoid confusion when trying to change VNAV ALT to VNAV PTH.
Other VNAV Considerations
It is worth considering what the FCTM 5.37 (note on bottom of page) recommends in regards to
setting of the MDA. If the FMC calculates a GP angle prior to the FAF, the pilot can consider
setting the MDA at the higher altitude. If there is no computed GP angle in the FMC prior to the
FAF, the pilot should wait until approximately 2nm before the FAF. In the KSEA approach the
pilot would set 2300’ approaching KLEWS and then set MDA approaching BUCKK.
Remember that the “D” on a LIDO chart refers to the Descent Point for the commencement of the
intermediate approach, not the Final Approach. The Final Approach Fix on a LIDO chart is
represented by an “F” whereas a Final Approach Point is represented by a “P”, for the purpose of
VNAV approaches it is acceptable to refer to the FAP as the FAF. (LIDO LAT 220, 230 and 410)
So why is VNAV SPD on approach an acceptable mode?
Although VNAV SPD is an acceptable mode during a VNAV approach it should only ever be a
temporary mode and usually engages after VNAV ALT or VNAV PTH, due to MCP
mismanagement or unexpected environmental conditions on the approach (tailwind is most
common).
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[FCOM 11-31-26…..”VNAV remains in VNAV PTH and follows the descant path unless the
airplane accelerates to within 5kts of the current flap placard speed and the airplane rises more
than 150’ above the path, in this case VNAV PTH changes to VNAV SPD]
The reason VNAV SPD is an acceptable mode is because you can only ever be above the
computed VNAV profile if VNAV SPD is displayed during an approach. It is impossible to be
below the profile “on approach” with VNAV SPD engaged. With VNAV SPD engaged a pilot
would need to manage the aircrafts energy (increase rate of descent whilst preventing speed
increase) until the profile is re established and VNAV PTH is automatically re engaged. Of course
if the FOM stabilization criteria could not be met by 1000’ AAL, a go around must be executed.
The VNAV SPD discussion leads us into the next misunderstanding.
Intercepting the profile from above.
Imagine this scenario: ATC has assigned you 5000’ as your final altitude and you are maintaining
5000’, yet the approach commences at 3000’. You are cleared for the approach and told to
“report established”. How would you manage this?
With VNAV ALT engaged no further action is required from pilot [except setting the MDA]
provided:
1) VNAV on approach criteria are met, and
2) VNAV is in the descent phase, and
3) The aircraft is below the computed VNAV descent profile
As alternative techniques you could select any other MCP Pitch mode other than VNAV (ALT or
VS 0/0) and then reengage VNAV [VNAV PTH will engage]. You could also type in 5000’ in the
FMC cruise page and press the altitude selector, VNAV PTH will engage but as we mentioned
earlier, you may have unintentionally deleted altitude constraints on the approach and additional
corrective actions will be required.