Altimeter vs the VSI

[PRB]

Ever since flying FS sim planes, I've used the VSI to “control” my altitude. The “quotes” are there to indicate that it has been less than successful. As I'm chasing the silly VSI needle up and down I am invariably “trending” up or down, and when I finally do look at the altimeter, I'm 200 feet off. I figured I just wasn't “doing it” right, and like a stubborn fool, did not alter my “technique”. Then I was reading a book, forget which one now, and there was a discussion about flight training. The instructor stressed that when maintaining altitude “... DO NOT use the VSI ...” (*gulp*) “ … use the altimeter! … “ (*duh*). The reason given was that the VSI lags, but the altimeter doesn't. And it works in FS too! It's much easier to maintain altitude while looking mostly at the altimeter, and quit staring at the VSI, thinking, as I did, that it will show “quicker” if you're going up or down. It may, but it doesn't help you maintain altitude.

Ok, so I thought it was interesting!

[Brett_Henderson]

It is interesting.. and just another aspect to simming that is very applicable to real flying.

For VFR flight, the vertical-speed indicator serves litle purpose beyond keeping you from subjecting your passengers to uncomfy climbs/descents. Every aspect of flight is primarlily about airspeed.. the resulting vertical speed is at most, a secondary reference. Like.. AFTER leveling off, it can alert you to a trend.. or AFTER estabishing a climb it can confirm a trend... but you don't use it for either.

Instrument flight is a different story. It becomes a vital cross-check when you can't see past the prop. But even then, it's mostly secondary. For example.. a known power setting and pitch will yield a known result. Your pitch instrument is the attitude indicator, and your manifold-pressure/tach are your power indicators... then the airspeed is referenced.. and then the altimeter. AFTER a climb or descent is established.. the vertical-speed indicator confirms the consistent trend that is expected of you.

[Lateral-G]

Your VSI is useful to set your rate of climb or descent. It comes in handy for determining how fast to come down in a given time over a certain distance.

Another use is for showing a trend in climb or descent. The altimeter reads static air pressure, which can vary as you fly due to atmospheric conditions. So, you could take off from one airport where the ambient air pressure is, say, 30.01", and you fly along at what you believe is a constant altitude and the ambient pressure changes to 29.02". Your altimeter should read a higher altitude (remember air pressure decreases as you increase in altitude) even though you flew with your VSI indicating 0 ft/min. Your VSI would tell you if you actually were climbing or descending.

It also is helpful so as not to make yours or your passenger's ears pop.

-G-

[Ken Stallings]

The VVI (Vertical Velocity Indicator) works by having a pin hole in a diaphram. The air pressure inside the diaphram reaches equilibrium with the outside air pressure in a few seconds.

However, if you change your altitude (up or down) the air pressure inside the diaphram will lag above or below the outside air pressure. If you climb, the air pressure lags ahead. If you dive, the air pressure lags behind.

The point is that because it takes a few second to reach equilibrium, the device is also terribly prone to instantaneous error. The VVI should only be used to measure change in altitude, and only then for a sustained climb or descent.

In instrument procedures, the VVI is never used to see if you are initiating a climb or descent. Instead, the two instruments you use for that are the altimeter and the artificial horizon. You only use the VVI in instrument procedures if the rate of climb or dive sustains itself over a few seconds. If you try to maintain altitude by referencing the VVI then you will enter a sustained oscillation up and down, chasing the VVI.

Furthermore, to make small correction in altitude while in instrument condtions, you use the artificial horizon and you do it by putting the dot one half a dot above or below the horizon line. I know that may sound silly (half a dot!!!) but that is exactly the kind of precision you need to fly with. Therefore, while maintaining constant altitude, your primary reference is the artificial horizon gauge. To change altitudes, the primary reference is initially the artificial horizon to enter the desired climb or descent angle, and then the altimeter to reach the desired new altitude.

Cheers,

Ken

[Brett_Henderson]

Your VSI is useful to set your rate of climb or descent. It comes in handy for determining how fast to come down in a given time over a certain distance

This is where simming crosses paths with reality. In the real world; by the time you're flying aircraft capable of altitudes where sustained, calculated rates of descent are needed.. you'd be answering posts like this The sim however, lets us find ourselves at FL330 before even understanding instrument flight..lol

In training type aircraft, or even some high-performance GA during VFR; your vertical-speed is secondary.. almost an after thought. For example: when climbing out, you don't shoot for a vertical speed.. you shoot for a climbing speed (Vx Vy, etc.). The vertical speed will be a function of that, mixed with load and density altitude. Even if you do find yourself trying to plan a descent in a light GA aircraft by vertical speed, any winds aloft will effect your ground-speed too much to make the effort worth-while. A descent is much like a climb. You set the power accordingly, and pitch(trim) for a desired airspeed and watch the altimeter unwind. The resulting vertical speed is near useless (aside from protecting passenger's ears.. )

[jmig]

In real world flying I always knew the "numbers." This power setting for this descent or climb rate. This A/S on final, etc. In the sim unless it is an aircraft I have flown a lot and memorized the numbers, I don't know them. so I tend to fly more by the seat of my pants.

I use the VVI only when on final to make sure I am descending at the proper rate. The average glide slope will bring you down around 500 fpm. Hold that on the VVI and you won't be embarrassed with one of the "Panther" landings because you touched down at 1500 fpm and collapsed the gear..

[fliger747]

The veracity and lag of instruments vary with aircraft and design. The inertial based instruments which run through a flight data computer are pretty much instantaneous in response, some aneroid instruments are not. In WWII many considered the VSI of the era not useful at all. Very fast aircraft are primaraly flown by attitude reference with a crosscheck of the altimeter. In the 747 we have a usefull little guy on the attitude display called a flight path vector, tells at a glance what flight path the airplane is making vertically or even in drift. Paste it on the horizon and keep it there and the altitude will stay the same....

Cheers: T

[TeaSea]

AAuuuughhhhh!!!!

Primary and Supporting!!

Pitch and Bank!!!!

Gyro and Vacuum!!!!


AAUUUUGGGGHHHH!!!!!



okay, guess who's studying for his instrument written.....

[Bjoern]

After reading this thread I got the feeling that I fly airliners all wrong.

I usually shoot for vertical climb speeds instead of airspeed for climbing. I don't even try to hold a certain speed, rather than getting the most fpm out of the aircraft to reach the upper layers (where jet engines are more efficient) more quickly.
I don't know how I got this habit, most probably after reading that lots of pilots not fully use their jet's climbing capabilities.

[fliger747]

The Auropilots in any default aircraft are not quite up to snuff when it comes to climb and airspeed if there is an autothrottle. The way we fly airliners? Varies. If we have an unrestricted climb to say FL 240, an intermediate leveloff and then another climb, we usually climb at full climb power at a favored airspeed for the weight and temperature, usually this transitions to a mach number at about 28,000 ft or so. In the 747 at a heavy weight we might climb at 350 knots IAS till reaching M .84 and then climb at that Mach.

However..... When making step climbs in RVSM airspace (1000 ft seperation for opposite aircraft) VS is often used to keep from the mode S transponders calculating a possible conflict between nearby aircraft.

Other times, for intermediate climbs (like when ATC gives a bunch of leveloffs) and you are carrying passengers, VS can be used (along with an airspeed) for passenger comfort. Big power and pitch changes make them nervous.

We often use a mode called Flight level Change, which brings up full climb power and maintains the selected airspeed through pitch. Hand flying I just adjust the pitch to maintain the desired airspeed/mach. In descent, the ideal is an idle descent, with pitch again used to maintain the desired airspeed or vertical path profile.

Cheers: T

[Bjoern]

So basically I can fly my jets the way I want to. Cool!

[PRB]

So basically I can fly my jets the way I want to. Cool!

Only the cargo ones! The boxes don't get "sceered" and they don't have ears to pop!

[Bjoern]

Only the cargo ones! The boxes don't get "sceered" and they don't have ears to pop!

Well, neither do my sim-pax. :d

[Lionheart]

Thanks Flieger. Thats really cool information. Good to know.

The test pilots at Epic aircraft were using speed as their max climb governor as well. So long as they were maintaining 175 knots, they could climb at nearly 3,000 FPM. Mind you, just a test pilot or two in the plane, almost empty.

Wild to hear about Mach. I have been away from tube liners too long.

Do you know if the A-380 has to do plateau climbs from being heavy with fuel? I have heard that you have to do this with the four seven when going for FL 400. Wondering if the A-380 has to also with those huge Trents..



Bill

[azflyboy]

Adding to what fliger747 and Ken Stallings said about the lag in older instruments, glass cockpit systems (like the G1000 in FSX) basically eliminate that lag, despite being hooked up to the same static lines an analog gauge would have been.

I flight instruct in Warriors and Cessna 172's with glass cockpits, and their VSI's register almost instantaneously, which is extremely handy for precise instrument flying, and makes doing steep turns within a 20ft altitude window incredibly simple.

[Ken Stallings]

In addition to using it to determine your descent rate for instrument approaches, the VVI is also used to rate climbs and dsecents while enroute. Normally, ATC wants a 500 feet per minute change in altitude rate so that they can plan the change to deconflict with surrounding traffic. If he says nothing in terms of a desired rate, the instrument pilot should assume 500 FPM.

I did not know the G1000 eliminated the lag in the VVI in GA aircraft. I knew about air data computers in the big jets, but this is the first I've learned that this can be obtained in GA aircraft -- nice to know!

As has been said, the primary method to achieve climb and descent speed (especially during instrument approaches) is to set a known power setting and trim for a desired descent rate, normally 500 FPM but that can vary depending up groundspeed (higher rate for a tailwind and lower rate for a headwind). Also, to smooth out your approaches, combine power changes with changes in descent rate. By doing this, you can add power concurrently with shallowing descent rate, and conversely retard power with increases in descent rate, and by doing this maintain your desired approach indicated airspeed.

What pilots strive to determine in new aircraft is the power settings needed to achieve various descent rates during an IAP while maintaining the desired approach airspeed.

Cheers,

Ken

[Bjoern]

Do you know if the A-380 has to do plateau climbs from being heavy with fuel? I have heard that you have to do this with the four seven when going for FL 400. Wondering if the A-380 has to also with those huge Trents..

Well, if you want to keep your speed up, probably.

If speed isn't an issue and you just shoot for FPM most probably no.


Two days ago I tried out Bill Leaming's Global Express. With a full fuel load, I had to step her up to FL510 in tiny, tiny steps with vertical speeds of 200fpm or less. It took *ages*.

[viking3]

A lot of airliners will step climb as their fuel burns down and then they can get up to where the winds/weather may be better. Mostly this is an issue with long haul ops because of the large fuel loads that must be carried.

Regards, Rob:ernae:

[vonernsk]

I understand from your post that you are attempting to hold your desired altitude by keeping the VSI needle level. This is an excellent method of flying level, but the key to it all is to trim the a/c out. This will limit the oscillations to an acceptable level.

Using the VSI needle to conduct level turns is a must and provided there is a quick response from the instrument, the VSI is a powerful handling tool.

Once the a/c is trimmed for a certain set of flt criteria ie alt and speed then the throttle becomes the means of ascent or descent.

I am not a trained pilot or professional aviator, just another fltsim nerd over a few decades, so all of the above might be 'balls' as we Brits say. Anyway thats my pennyworth and my first reply to a thread ever!

Good luck.

[Ken Stallings]

Sorry, but I have to correct what you said.

You absolutely do not use the VSI to ensure your turn is level. Instead, you use the artificial horizon combined with the altimeter. Of the two, while you are in the turn, you reference the altimeter more. If you are IMC (in the clouds) and cannot reference the horizon, you must keep the altimeter in your cross check while in a turn and you focus primarily on the directional gyro (DG) because you have to lead your roll out so you don't overshoot your intended new heading.

While in a turn, the VSI is notoriously unreliable. Now, perhaps in very expensive jets with air data computers (ADC) the VSI may not lag. But even then your eyes can only focus on so many inputs during a turn. You have to reference the DG primarily, and back it up with the altimeter to ensure you keep your altitude plus reference the turn coordinator so that you can maintain a standard rate turn while IMC.

An ADC can take inputs and stabilize them by excising out spurious inputs. However, it is also an issue of the order you use the instruments in an IMC turn. First you want to reference the turn coordinator to ensure you are at standard rate. Second, you want to transition to the DG to monitor the heading to lead the roll out. Periodically, you want to reference the altimeter to ensure you are maintaining the desired altitude while in the turn.

If you need to change pitch to correct an observed altitude change (by reference to the altimeter) then you reference the artificial horizon to change the pitch setting to correct. You strive during instrument flight to keep the change of pitch around a half a dot width.

There is a difference between long term change in altitude versus small immediate corrections. While in a level turn, making the small corrections does not require changes in power but merely a quick reference and very small (half dot width) adjustment to the pitch as referenced on the AH. You use pitch and power to make sustained changes in altitude to remain on glideslope during an instrument approach. And like I wrote, the smoothest way to do it then is to combine pitch with changes in power to maintain desired final approach airspeed.

Cheers,

Ken