The MCP has several features that can be used to control the outputs from the auto pilot: Level Change (LVL CHG), Vertical Speed (V/S), Control Wheel Steering (CWS), Lateral Navigation (LNAV), Vertical Navigation (VNAV), Speed Intervention (SPD INTV) and Altitude Intervention (ALT INTV).
In this post we will examine the use of altitude and speed intervention and demonstrate the use of these modes in flight with VNAV and LNAV engaged. A breif recap of VNAV is provided, however, for the most part it is assumed you already know the basics of what VNAV is, and what it can do.
Before continuing, you must realize that there is a lot of interconnection between the differing avionics of the 737 aircraft. When you alter one aspect, often a corresponding change within another system may occur. These systems are replicated in the flight avionics suite that you are using, however, functionality is often dependent upon which software suite you are using. The following information pertains only to ProSim737’s avionics suite.
Finally, be aware that not every Boeing 737-800 has the same functionality. Speed and altitude intervention are company options that may or may not be ordered at the time of airframe purchase. The acronyms FMC and CDU are interchangeable in this post.
VNAV can be confusing
Understanding VNAV and the use of Speed and Altitude Intervention can often be a confusing subject to grasp; real-world pilots with years of training can become perplexed to how the flight computer is controlling the aircraft. A report commissioned by Honeywell (Cognitive Engineering Anaylsis of Vertical Navigation, 2000) indicated that "the use of VNAV are well known sources of operator confusion, and explain, in part, the operational issues experienced by airline pilots".
I've attempted to convey this information succinctly; however, please treat this as a introduction. I recommend reading the appropriate section in the Flight Crew Operations Manual (FCOM) for a more thorough understanding (see bottom of page for link to download).
VNAV, MCP & FMA
An often misunderstood facet of the MCP is that the illuminated lights indicate whether a function/mode is turned on or off. This is not entirely correct. Illuminated lights indicate modes that can be switched off. Active modes that cannot be switched off, extinguish their light on the MCP. A pilot (virtual or otherwise) would be foolhardy to rely solely on this method to determine the operational status of a mode or function and cross referencing with the FMA and CDU is considered wise.
The Flight Mode Annunciator (FMA) seen above the Primary Flight Display (PFD) displays various alerts and status messages and it’s prudent to cross reference between the FMA, MCP and CDU to determine what exactly is happening at any given time. When a function is about to be executed, the FMA will draw a green rectangle around the function.
- If in doubt, always disengage VNAV and then reengage after studying your predicament.
Speed Intervention (SPD INTV) & Altitude Intervention (ALT INTV) – Why?
The flight deck can be an extreme work environment, especially during the high-task descent approach phase of the flight; speed and altitude intervention were designed to allow pilots to easily and quickly change either the speed or altitude of their aircraft without re-programming the CDU or disengaging VNAV. The intervention buttons are strategically located on the MCP and when engaged (pressed) allow you to quickly change either speed or altitude level on "the fly". To engage these buttons they must be depressed for one to two seconds.
You are flying at FL280 (28,000’) at 300 kias (.54 Mach). The CDU has a flight plan engaged (Company Route) and the CDU LEGS page specifies speed and altitude constraints. VNAV, LNAV and CMD (A or B) is engaged. The speed window located on the MCP should be blank (one indication that VNAV is engaged) and the VNAV, L-NAV and CMD A or B indication buttons will be illuminated.
LNAV will be controlling the lateral navigation of the aircraft while VNAV will be controlling the altitude of the aircraft relative to the altitude and speed constraints that have been set in the CDU.
You have been asked by ATC to decrease your speed to 280 kias and lower your altitude to FL260.
Speed Intervention (SPD INTV)
Engaging speed intervention you will immediately observe that the MCP speed window becomes active iand displays your current speed. Dial in into the speed window the new speed requirement of 280 kias. Notice on the PFD that the speed indicator value above the speed tape has changed from 300 kias to the new speed of 280 kias. If you cross check with the cruise altitude (INDEX/PERF INT/CRZ ALT) in the CDU it will still indicate the cruise speed of 300 kias.
If you wish to stay at this speed (280 kias), you will need to manually alter the cruise speed constraint in the CDU. However, in this case the reduction in speed is momentary and ATC advise you to return to your original speed.
To do this, you press the SPD INTV button and your speed will return to the original speed. Note the speed indication on the PFD changes from 280 kias back to 300 kias. Also note that the MCP speed window is now blank (no actual speed display).
If you practice this a few times, it will become second nature and will soon be your preferred method to alter speed momentarily, but maintaining the original speed constraint within the Vertical Navigation system.
Altitude Intervention is little more convoluted compared to the simpler Speed Intervention. This is because the relationship changes between whether the aircraft is ascending or descending.
In normal flight with VNAV and LNAV engaged the FMA will read FMC SPD / LNAV / VNAV PTH and the V-NAV, LNAV and CMD buttons on the MCP will be illuminated.
Descent using ALT INTV - Let’s assume the same scenario (FL280 to FL260)
Dial into the MCP altitude window the new altitude (FL260). You will observe the PFD changes from the original altitude of FL280 to FL260. Press the ALT INTV button and you will notice the FMA annunciates: FMC SPD / LNAV / VNAV PTH. The aircraft will then descend at 1000 fpm until FL260 is reached.
It’s a function of ALT-INTV that all descents are maintained at 1000 feet per minute. Interestingly, this corresponds to the same decent rate when using “Descend Now” in the CDU.
If you cross check with the cruise altitude (INDEX/PERF INT/CRZ ALT) in the CDU it will still show the original cruise altitude of FL280. The CDU has NOT been automatically updated to the lower altitude (this is normal).
To return to the original altitude level of FL280, dial into the MCP the previous altitude level (FL280), press ALT INTV and the aircraft will ascend to FL280.
However, if you want to remain at FL260, you will need to manually update the cruise altitude in the CDU.
- It’s important to remember that VNAV will not engage unless the original altitude level (FL280) is manually dialed into the altitude window of the MCP.
Depending upon company procedure, the usual method to accomplish a cruise descent in VNAV is to dial into the MCP altitude window the lower altitude, then open the the DES (descent) page on the CDU. Follow through by typing the new altitude into the scratch pad and by pressing CRZ DES (cruise descent). The aircraft will begin a descent at 1000 feet per minute whilst maintaining VNAV. The LEGS page and the PERT INT altitude in the CDU will also be updated to reflect the altitude change.
ASCENT using ALT INTV - Let’s assume the same scenario (FL260 – FL280)
The ALT INTV button operates a little differently when you ascend. For a start, it automatically updates the cruise altitude (CRZ ALT) to the new altitude level. The FMA will annunciate: N1 / LNAV / VNAV SPD during the climb phase of the flight, changing to FMC SPD / LNAV / VNAV PTH when the new altitude level is reached.
- It’s important to remember the main difference between using ALT INTV during an ascent or decent is the Cruise Altitude in the CDU ONLY updates for ascents and not descents.
Special Considerations – ALT INTV
When using ALT INTV, there are several variables pertaining to the altitude constraint that alter depending upon whether you are in VNAV climb, cruise or descent. Rather than re-word what already has been written, I’ve scanned the appropriate page from the Cockpit Companion by BIll Bulfer below.
Other Ways to Change Altitude Maintaining VNAV
There are other ways to either ascend or descend to an assigned altitude and still use VNAV. You can engage Level Change (LVL CHG) or use Vertical Speed (V/S); using either will cause the aircraft to descend. Level Change will cause a relatively fast descent whilst Vertical Speed allows you to input the descent rate – both of which can be greater than the 1000 fpm ALT INTV uses.
Dial into the MCP altitude window the new altitude level and press either LVL CHG or V/S and the aircraft will descend to that altitude. Note that the V-NAV light on the MCP extinguishes and the speed window becomes active showing the aircraft’s current speed. To return to the original altitude level, dial into the altitude window the original altitude level and press the V-NAV button on the MCP.
I prefer to use LVL CHG or V/S to initiate a descent to be more reliable than using the ALT INTV (read on…..).
Reliability of ALT INTV – ProSim737
In ProSim737 using Version 1.27, I've observed that the ALT INTV occasionally exhibits difficulty in holding a lower altitude level.
The Boeing system is designed in a way that once the V-Path is intercepted, the Flight Director cross hairs maintain the new altitude by pitch. In ProSim737 this pitch is often difficult to hold and a resultant pitching of the aircraft (up and down) occurs as the system attempts to hold altitude. When using Level Change (LVL CHG) or Vertical Speed (V/S) this does not occur.
I’m unsure if this behavior is common only to my system or is more widespread; but a way to solve the issue is to open the CDU (Index/Perf/Cruise Altitude) and change the cruise altitude to your new height. Another method is to deselect V-NAV completely. If and when you wish to return to a higher altitude, dial in the higher alttiude increment into the MCP and then select either VNAV or ALT INTV. The aircraft will return to that altitude and V-NAV will be active. The CDU will automatically update its cruise altitude to the new height as well as updating the alttiude in the LEGS page.
I find that V-NAV can cause confusion, especially if you are still learning the system and how it interacts with other FMS variables.
- If at anytime during a flight you observe a VNAV discrepancy and become unsure of what the aircraft computer is doing, the best option is to turn VNAV off and on again - usually this solves any potential problem.
The developers at ProSim737 are continually tweaking these variables. In future software releases this issue may well be rectified.
During a flight there are many of reasons why you may need to alter speed and/or altitude; whether it is to divert around a localized weather pattern or to abide by an Air Traffic Control request. Whatever the reason, often the new requirements are short-lived and a return to the original constraints necessary.
Both the SPD INTV and ALT INTV functions can be used as time savers to alter either speed or altitude without spending unnecessary time updating the CDU. The time saved in the latter being especially important during the high-task descent and approach phase of the flight.
In this post I've attempted to explain the intervention functionality of the B737 and provide a "work-around" should V-NAV not operate as anticipated. However, to fully understand the interrelationship between these systems, the CDU and flight dynamics I strongly recommend perusing the Flight Crew Operations Manual (FCOM) and the “Cockpit Companion” written by retired aircraft captain Bill Bulfer.
If I’ve made any glaringly obvious mistakes in this post, please contact me so they can be rectified.
To read a review of the Cockpit Companion.
To download the Flight Crew Operations Manual (FCOM) navigate to the documents section where there are several versions for download.
Table 1: ALT-INTV FMA indications and notes
FMA – Flight Mode Annunciator
VNAV – Vertical Navigation
LNAV – Lateral Navigation
MCP – Mode Control Panel
ALT INTV - Altitude Intervention
SPD INTV - Speed Intervention
LVL CHG – Level Change
V/S – Vertical Speed
CDU – Control Display Unit
FMC – Flight Management Computer
PFD – Primary Flight Display
FMS - Flight Management System
Thanks to those readers who have e-mailed recommending suggestions and pointing out a few glaringly obvious spelling mistakes which crept into this post. Cheers, F2A
An interesting thread on the AVSIM PMDG 737 NGX forum that discusses the valid uses of VNAV during approach. Click here to be directed to the forum.