How Does Autopilot Determine Rate of Climb / Decent

[lehbird 55]

So when you allow the autopilot to determine the rate of climb / decent how does it determine the rate? Case in point, when I fly the Fenix or the PMDG 737 and go VNAV, the rate climb / decent can sometimes be 3k-4k FPM. This isn't realistic or appropriate, is it? 

Is there something I should be adjusting in the FMC to control this?

Thanks,

Dave 

[ATRguy 94]

No. It climbs on a preset speed and power setting, so if you’re light you get a good rate.  If you’re heavy, you don’t.  The airplane is pressurized, it doesn’t really care about the climb rate.

[lehbird 55]

4 minutes ago, ATRguy said:

No. It climbs on a preset speed and power setting, so if you’re light you get a good rate.  If you’re heavy, you don’t.  The airplane is pressurized, it doesn’t really care about the climb rate.

Thanks. But is that realistic? Do airliners really climb / decent that steeply? I would guess no, but could be wrong. 

[ATRguy 94]

Just now, lehbird said:

Thanks. But is that realistic? Do airliners really climb / decent that steeply? I would guess no, but could be wrong. 

They certainly do if given the opportunity.  
 

Sometimes ATC restrictions and climb thrust derating will change that, but yes generally aircraft climb quickly so they get up high and burn less fuel.  
 

At my airline, in the ATR42 we climb at 160 knots (whatever rate the gives us) and descend at 2000-2500 fpm so we burn less fuel in the descent.  

[JYW 2,990]

Good explanations from ATRguy.

VNAV is using airspeeds as it's reference.  Then setting the autrothrottle to a fixed power level.  The aircraft will then adjust the pitch of the aircraft, to maintain that given speed that it's chasing.

In other words, VNAV is using Flight Level Change, rather than Vertical Speed.... and this is why you will see varying rates of vertical speed on a VNAV climb.

[lehbird 55]

28 minutes ago, ATRguy said:

They certainly do if given the opportunity.  
 

Sometimes ATC restrictions and climb thrust derating will change that, but yes generally aircraft climb quickly so they get up high and burn less fuel.  
 

At my airline, in the ATR42 we climb at 160 knots (whatever rate the gives us) and descend at 2000-2500 fpm so we burn less fuel in the descent.  

Thank you. So a climb / decent rate of 4k-5k FPM is not "uncomfortable" for passengers and is permitted by airlines? In some airline simulation products I use I loose points for those rates of climb / descents so I figured they probably were not appropriate in the real world. 

[Farlis 2,331]

18 minutes ago, lehbird said:

FPM is not "uncomfortable" for passengers and is permitted by airlines?

I'd say that depends on how good the pressurization of the particular aircraft is.

As long as it manages to maintain a climb or descent rate below 1000ft per minute, it matters little how fast you actually descent or climb.

[lehbird 55]

2 minutes ago, Farlis said:

 As long as it manages to maintain a climb or descent rate below 1000ft per minute, it matters little how fast you actually descent or climb.

Yes, but I am talking about climbing at 4,000-5,000 FPM which is 4-5 times that. 

[Farlis 2,331]

1 minute ago, lehbird said:

Yes, but I am talking about climbing at 4,000-5,000 FPM which is 4-5 times that. 

So what? It's the pressure system that determines how much you can feel of that. In some aircraft it is better at adapting, in others it isn't.

[Luis Hernandez 594]

@lehbird, what Farlis meant is as long as it manages to mantain a cabin altitude climb or descent rate below 1000 ft/min. If you can see the 737 overhead, there's some kind of VSI in the pressurization section. This is the one that you want to keep between -1000 ft/min and +1000 ft/min.

If the pressurization system is good, it won't matter what vertical speed you see in your PFD.

Edited February 23 by Luis Hernandez

[Bob Scott 7,760]

In my r/w Gulfstream flying, when departing with a light jet, after clearing any obstacles and at/above altitude restrictions I'd fly the climb in VS mode at 3000 fpm until the power required to hold that rate was close to the FMS computed climb power, and then switch to VNAV for the rest of the climb.  It's not just about cabin pressurization--at light weights, climb rates at full climb power could be 5000 fpm or more, which results in an uncomfortably high deck angle. 

The Gulfstream V could carry enough fuel for a 12-hour leg, but most of my flights were far shorter, so really lightweight departures were common.  I suspect it's less often an issue with airliners, as they tend to fly pretty full of pax these days, but something like a short leg in a 777 with a super light fuel load for a 1-2 hour leg (Air France used to fly Santiago Chile to Buenos Aires enroute to Paris as one real-world example) would pose the same considerations.

[ckyliu 1,749]

2 hours ago, lehbird said:

a climb / decent rate of 4k-5k FPM ... In some airline simulation products I use I loose points for those rates of climb / descents so I figured they probably were not appropriate in the real world. 

That says more about those "airline simulation products" then it does about Fenix or PMDG aircraft. I have often questioned the arbitrary nature of the judgements/scoring implemented and that's why I don't use them.

Edited February 23 by ckyliu

[G550flyer 1,052]

2 hours ago, lehbird said:

Yes, but I am talking about climbing at 4,000-5,000 FPM which is 4-5 times that. 

There is a balance between requirements and passenger comfort. All depends on your situation. At takeoff, the goal is to get up and away from the ground quickly. I want to do that as expeditiously as I can. If there is mountainous terrain or I'm weight limited, I'm going full rated power and I will take what ever V2+10 etc gives me. If it's flat all around and no climb requirements, I can derate and save on the engines. With that logic, at times you will be 4000 to 5000 FPM after lift off and I've done that. The flight manual may say, no higher than 25 degrees or 20 degrees for take off and you set it, if you get there and let her accel. The key point here is if you experience an issue, where do you want to be? If I lost both engines, I definitely want to be higher to have more options. We already plan on an engine loss, even then, I'll take height, especially near mountainous terrain.    

Now, as Bob pointed out, there are some techniques for passenger comfort. Once we are clear of terrain and requirements, we can focus more on passenger comfort. As he stated, I can use 2000 to 3000 FPM to reduce the deck angel. I can also do the same when configuring for approach. The DC10 was notorious for having about a 13 degree deck angle at min maneuver with only the slats out. I can burn a little more fuel with flaps 15 and get the nose closer to 5 degrees. Another technique is in the use of FLCH/IAS in climb. Instead of punching FLCH/IAS and letting the throttles surge forward and the nose to pitch for the moon, you dial up the V/S speed slowly until the engines reach climb power and then engage FLCH/IAS. The passengers will barely notice. Another area is speedbrake use. An older guy once told me that using the speedbrakes is like easing out and back into bed without waking your spouse. You ease them out and when you are done, you ease them back in. There is no need to rip them out and back in jolting the whole jet and scaring folks.

So to answer your question, yes. Depending on the aircraft and it's power, you may find yourself climbing for the heavens at 4000 to 5000 FPM and it's completely normal. 

[Farlis 2,331]

2 hours ago, Luis Hernandez said:

@lehbird, what Farlis meant is as long as it manages to mantain a cabin altitude climb or descent rate below 1000 ft/min. If you can see the 737 overhead, there's some kind of VSI in the pressurization section. This is the one that you want to keep between -1000 ft/min and +1000 ft/min.

If the pressurization system is good, it won't matter what vertical speed you see in your PFD.

Indeed. This has made quite some improvements over the years. I have flown very rarely in real life but I remember my first transantlantic flight in the 90's in a Tristar. The descent was murderous for the ears. Then I didn'f fly long haul for about 15 years and the next flight was on a 747-400. I expected my ears to pop like crazy again and felt virtually nothing.

[Bob Scott 7,760]

15 minutes ago, G550flyer said:

There is a balance between requirements and passenger comfort. All depends on your situation. At takeoff, the goal is to get up and away from the ground quickly. I want to do that as expeditiously as I can. If there is mountainous terrain or I'm weight limited, I'm going full rated power and I will take what ever V2+10 etc gives me. If it's flat all around and no climb requirements, I can derate and save on the engines. With that logic, at times you will be 4000 to 5000 FPM after lift off and I've done that. The flight manual may say, no higher than 25 degrees or 20 degrees for take off and you set it, if you get there and let her accel. The key point here is if you experience an issue, where do you want to be? If I lost both engines, I definitely want to be higher to have more options. We already plan on an engine loss, even then, I'll take height, especially near mountainous terrain.    

Now, as Bob pointed out, there are some techniques for passenger comfort. Once we are clear of terrain and requirements, we can focus more on passenger comfort. As he stated, I can use 2000 to 3000 FPM to reduce the deck angel. I can also do the same when configuring for approach. The DC10 was notorious for having about a 13 degree deck angle at min maneuver with only the slats out. I can burn a little more fuel with flaps 15 and get the nose closer to 5 degrees. Another technique is in the use of FLCH/IAS in climb. Instead of punching FLCH/IAS and letting the throttles surge forward and the nose to pitch for the moon, you dial up the V/S speed slowly until the engines reach climb power and then engage FLCH/IAS. The passengers will barely notice. Another area is speedbrake use. An older guy once told me that using the speedbrakes is like easing out and back into bed without waking your spouse. You ease them out and when you are done, you ease them back in. There is no need to rip them out and back in jolting the whole jet and scaring folks.

So to answer your question, yes. Depending on the aircraft and it's power, you may find yourself climbing for the heavens at 4000 to 5000 FPM and it's completely normal. 

"Safety-Comfort-Reliability"...SAMFOX, baby!!  😉