question on descent

[steve1975]

I have been watching the videos and I use Vnav to climb but on descent I notice that people use V/s and disengage the vnav. I believe I heard Jan say in one of the videos in real plane its unrealable. My question is do you use the same info the FMC gave for waypoints speeds and height. If no how do you decide which speeds . I am a newbie and know under 10000 its 250 or less. but I am guessing 220 to put flaps down but how far out and height. 

Is there videos that deal with these things. I am sure they do them in the full flights but I sometimes miss things.

thanks

[Iain]

Hi Steve

There is no black and white answer to your question.  I use a few general rules, and I use various methods for descent.

VNAV descent works fine for me, the initial descent from cruise can be quite steep, the throttles will slow the aircraft down slightly to descent mach number, then the aircraft pitches down to maintain speed, what I see is on the pitch down, the speed is often slightly low, so the initial descent can go up to -6000 ft per min for about 10 seconds, then it corrects itslef to a more normal level.  I don't know if this is what the real aircraft does, and thats what Jan is referring to.  As far as I know, a common way to descent (in real life) and the way I do it is to select V/S mode about 10nm before TOD, select -500ft, then -1000ft, then as you start to intercept the descent path, you can select VNAV and it should result in a more gradual descent.  If you ever descent in V/S all the way down, be aware to change the MCP from mach to KTS at the typical cross over altitude,  26'000ft .  You don't want to have a high mach number in the MCP at low altitudes. It's handy to understand how VNAV switches from targeting a speed in kts, to targeting a speed in mach. You can search mach cross over speed to learn more.

For descent speed, the FMC will tell you the target descent mach, typically slightly under the cruise mach. This is a good speed to use for your initial descent.

For heights, there are a few things you need to check.  First is check the STAR you are flying, there might be height restrictions in that.  If you have selected the STAR in the FMC, the restrictions should already be there, and can be seen in the LEGS page of the FMC.  It's a good idea to check these before you even takeoff, put the FMC in LEGS page, put the ND in PLAN mode and then you can see right side lower select key in now a STEP function, you can step through your whole flight plan and see all waypoints, with speed and height restrictions if any showing on the right hand side of the FMC.  Getting this plan correct is key to a smooth flight.

However, you can also fly all the speed and height restrictions with no help from the FMC.  You should always be aware of where your TOD should be, this is also important for the IXEG 737 as the FMC can sometimes throw a wobbly and give you TOD 500nm from your destination so it's good to be aware of that.

A general rule is to mutiple your cruise alt by x3, then use that as the number of NM for your descent, so if you are at 36'000ft, and you want to descent to 0ft, multiply 36 x 3, start your descent 108nm away.

For height restrictions, check the STAR, there may be restrictions like cross TLA at FL070, so if you are at 32'000ft, and you want to cross TLA FL070, then thats a descent of 25'000ft, multiple 25x3 and you should start your descent 75nm from TLA.

A good feature of the aircrtaft is the green arc, when you descend, the green arc shows you at what point you will reach your altitude you have selected in your MCP.  A very powerful tool, so if you get a cross TLA at 070 restriction (from the STAR or from ATC instruction, just put 7000ft into the MCP, then set the V/S until you see the green arc over TLA, then you know you will reach TLA at 7000ft, if the arc is a bit beyond TLA, just increase your V/S slightly.

For speed restrictions, 250kts below 10'000ft generally, this speed restriction is often broken in area's where there are not so many aircraft, although in most area's it's a hard restriction.  I use FR24 to see what typical speeds are below 10'000ft.  Taking the 737 back from the USA with Norwegian, above 250kts on climb out is often ok, same in Scotland where pilots will often ask for any speed restrictions and will be told negative, so I descend at 280kts below 10'00ft.  Just one notes, try NOT to play around with speed restrictions in the FMC once you are in the air, this for me seems to be the most common cause of the VNAV function to go crazy and give you incorrect TOD's.  Set up any speed restriction before takeoff.   Use the CLB page, I typically set the 280/ the CLB page here.

Lastly, remember you also have LVL CHG button, this is very handy to use if you want to descent with idle thrust (and th VNAV has gone a bit crazy), you can use LVL CHG, you just need to be aware the this mode is ignoring any height restrictions and speed restrictions from the FMC, so you need to monitor that yourself.  

Cheers

 

 

 

 

 

Edited August 16, 2018 by Iain

[mgeiss]

Iain already gave an elaborate answer, but here is a tl;dr of how I do descents depending on the situation:

LVL CHG when I just can get down to the next altitude without needing to obey any restrictions on the way. E.g. when ATC says "descent and maintain XYZ".

V/S when I want to reach the next altitude at a given point. E.g. when approaching an altitude restricion on a STAR or when ATC says "cross ABCDE at and maintain XYZ".

 

V/S can also come in handy when there is a speed restriction coming along the way. E.g. you are going to descent through 10000ft and you want to keep descending at a reasonable rate while also needing bleed off speed, you could dial in -1000 ft/min and 250knots. When the plane has slowed down to 250, you could then switch over to LVL CHG to expedite the descent.

Edited August 16, 2018 by mgeiss

[steve1975]

Thanks those are really helpful will

praxtixe this weekend . One of the things that thru me of was after plane was to descend the first waypoint on the decend has a higher speed than then cruise which I didn’t notice when checking the legs . I don’t think it was Mach but maybe

thanks again . Are there any videos you recommend 

[steve1975]

Just wondering in general normal cruise speeds in knots . Not ground speed . Over 30k ft or 26 to 30 or 20-25. I think iain said over 26k goes to Mach 

[Iain]

I think climb speeds are about 300kts then at 26k it switches to Mach, typical cruise is Mach 0.76 or there abouts, she is an aged lady so can’t go as fast as modern chicks.  I would recommend learning the fmc as much as possible as it will set the correct Mach for you, it will tell you your most optimal cruise height for your weight etc. it can be daunting but it’s worth watching some tutorials on it.

[steve1975]

having a few questions about speed and altitude, I am trying to land in KLAS on runway 26R, and using the KEPEC6 approach. there are lots of specific heights not far from each other . When I am decending would I put the alt button on the KEPEC at 13000 ft and play with speed then nipzo at 9000 etc. Would I keep changing the Alt hold button for each of these as a new flyer seems like a lot on work for one person. Of if you would just put the glide scope capture alt and hand fly the plane keeping it at those altitudes 

[Iain]

Looking at the chart on Navigraph, if you using the MISEN transition, then you can see some hard speed and altitude restrictions.  MISEN FL240, CLARR 13'000ft & 250kts, (now called feet as your change from FL to ft at 18'000ft in the USA), KPEC 13'000ft etc etc.

There are a few ways to fly it, if (and you should) have your FMC setup before takeoff, and have the expected STAR and RW already planned (better using 26L for this approach), then your FMC will show all these altitude and speed restrictions already.  While cruising along at whatever height, just before TOD, set your MCP height down to 3800ft (the ILS height of 26L), then your VNAV will descend you all the way to the ILS and follow all the restrictions for you.  You just need to check it's behaving, put your landing light on, have a cup of tea!    Where you see the small black writing for heights next to the lines, those are the minimum height, don't try and fly at those heights!

If ATC gave you no instructions, then you can let VNAV do it all for you, probably more realistic is to set your MCP atl to 24000, and when you get to it, change to 13000, then it's up to you to press V/S and set a V/S so the green arc is over CLARR showing you will reach CLARR at 13000ft, or you can just set 13000 and press LVL CHG, and it will descent you at idle thrust, in this case, you may reach 13000 before you get to CLARR but that's ok.

That's one of the main things with sim flying, you can VNAV the whole descent as you have no ATC to give instructions, or you can do it a bit more manually, and pretend that ATC are telling you to descend, and then you do it with V/S or LVL CHG. 

Watch some youtube videos, loads of cockpit video's out there, plenty of 737-800 etc, a few with the -400, you can see how it works in real life.

My favourites here, cargo 737.

https://www.youtube.com/watch?v=Z49i6vpM5Is&t=2640s

https://www.youtube.com/watch?v=lztqIM0aTTs

 

 

 

Edited August 20, 2018 by Iain

[steve1975]

Thanks, trying tondeacent without vnav. But seemed a lot with v/s changing alt at each waypoint speed etc . For one person with not a lot of time between each waypoint .

[Iain]

Your right in that being single pilot can make the cockpit quite busy, there are a few things you can do to make sure you have enough time for everything.  I am guessing you are flying offline so you don't need to worry about ATC.

Make sure you use the checklists, or like me, I made my own checklist as simming has some differences to the real world.

For the descent, check the WX for your destination before you reach the TOD, have all the RW and ILS info setup in the MCP and FMC before you reach your TOD, have pressurisation and air con sorted before TOD

Use VNAV for the initial descent, it will descend you and follow the altitude and speed restrictions as you descend, you only need to make sure you have the MCP height updated, in real life you would follow descent instruction, but in the sim you can dial in 2000ft to the MCP before TOD, and then just monitor it.

Be ready for the 250kts speed restriction, at 12'000ft, have the MCP mode ready for the speed reduction to 250kts, be ready with landing lights.

For waypoint heights, a good tip is to not try to reach that waypoint at that height, the restriction doesn't mean reach the height jsut as you get to it, it only means be at the height when you finally get to it, so if you are at 16'000ft and the next restriction is 12'000ft and it's 20nm away, you can press LVL CHG, let the aircraft descent at idle thrust, you will reach 12'000ft a few minutes before you get to the waypoint but that's ok, it gives you time to monitor your approach, check weather, get ready for the next descent, manage your speed etc.

 

The 737 doesn't go down and slow down very easily so it's a good idea to only do one at a time, ie. descent at a certain speed, or slow down when level, it's not easy to descent and slow down.

[steve1975]

thanks again, I do put the star in the FMC, but everytime I try a vnav descent something goes wrong, one time I forgot to adjust the alt hold button from cruise height, and another couple times the vnav would not change the speeds according to the fmc. 

 Any advice , I want to fly my trip I took a bout 6 weeks ago from Toronto to Liberia , costa Rica. I is a 5 hour flight and was in a nightmare A319( 6'3" and 275lbs and my knees were jammed into the seat). Its about a 5 hour flight. What do you do when doing long flights. I don't want to leave in case the flight goes crazy or miss top of descent. 

[Iain]

Well, if you want to simulate the flight conditions, jam your chair up against your desk till your knees are crushed and sit there the whole flight!

But seriously, there are a few ways to plan for a flight.

- Just fly a straight line,  put departure airport and destination into the FMC takeoff, head towards destination and land. Less planning, less FMC work etc, but highly unrealistic.

- Plan the route yourself, use something like Skyvector.  Go to Skyvector, select flight plan, put in CYYZ - MRLB, set map option to World Hi, then start to join up the flight plan yourself, very time consuming.

- A more popular method is to use and online flight planner to generator the flight plan for you.  My VA generates my flight plans, which i then modify, you can use free web sites such as 

http://rfinder.asalink.net/free/

Put in CYYZ-MRLB and hit Find Route, it then generates the route below.  You can then copy paste this into skyvector (the white box in the flight planning section) and now you can see your full flight plan below.

CYYZ SID FOXEE Q145 HVQ J85 SPA J53 IRQ J81 GRADY J53 DUNKN J81 NOWAY J53 CRG J113 DEARY J20 LLNCH J113 ARKES J20 VKZ G430 EONNS A509 URSUS UA301 UCA UB767 GCM UG877 MGA UM328 IMOLA STAR MRLB

You could put this flight plan into the RTE page of the FMC manually, ie. to HVQ via Q145 to SPA via J85 etc etc, but this can take a while for a long route.  I do this method for routes that are not too long, the route above would only take 5 mins to input though.

There are plenty other websites that generate routes, the other one I use is Simbrief, the advantage of this is that it generates the flight plan, and you can then save this file to a folder on your pc then move it into the co-routes folder and load it directly from the FMC.  You then only have to select runways, SIDS and STARs and off you go.  The website also gives you full fuel planning etc, the only thing you have to do is manually manually register your aircraft (weights etc) one time as the IXEG 737 isn't there by default.

For you FMC problems, VNAV errors can be minimized by checking the FMC flight plan before you start engines.  For me this is VERY important as you can have a smooth flight 99% of the times if you do this...and I have done about 400 flights in this aircraft.  To do this, finish your whole FMC planning, make sure you have no errors, then check the legs page and make sure you have a speed and altitude in the right hand side for EVERY waypoint.  This is what the FMC is following, if any line is blank, it will screw up. The most common error with FMC planning is pressing EXEC to activate the route, but forgetting to press EXEC on the performance page and then the LEGS page is not correct.

Long flight are quite easy to plan, PMDG have a nice feature called pause at TOD, which doesn't exits in the IXEG plan, but, since the TOD can become a bit messed up, you should try using a programme called Autopause or pauseforme.

https://forums.x-plane.org/index.php?/files/file/7257-autopause140zip/

At anytime, go to the plugin from the menu, select the airport box, type in the ICAO code, put in the nm you want it to pause, activate it, close it and leave your sim alone.  The majority of decents start between 130 to 100nm away at normal cruise heights so set this to 150nm or 200nm then you have time to sort yourself out when you get back to your sim.

Hope that helps.

 

 

Edited August 29, 2018 by Iain

[steve1975]

Thanks , I wasn’t clear sorry . I don’t have trouble making flight plans . I usually do run through the legs which is why I get frustrated but it happens . 

When I talked about long flights sorry it wasn’t about pausing it was just it’s so long with nothing to do in the cockpit while cruising . It you watch a shoe in Canada cslled W5 it talks about pilots taking snoozes etc . Canada has some of the worst rest to work ratios in the world for pilots