F27 Autopilot "Airspeed hold"

[Alex_Ford]

Hello Everybody. I have a question ref the/a F27 Autopilot. It has a control marked "Airspeed" which says that it 'automatically maintains the indicated airspeed'. My question is how does it do that as I see nothing in the airline operations manual and AP diagram in it that links the AP to the engines, only to the elevators and pitch trim on them (plus rudder and ailerons). With that in mind how can the AP 'maintain the indicated ASI' unless it uses the pitch trim which would result in climb or descent which is obviously not wanted. ??

Thanks in advance.

[STBYRUD]

Hi Alex, why wouldn't that be wanted? I suspect this works the same as level change / open descent on Boeings and Airbii, general aviation autopilots sometimes call this 'IAS' mode. Airspeed is kept with pitch commands, nothing else. A lot safer to climb (and descend) at fixed power setting and speed than with a selected vertical speed!

[Alex_Ford]

Thanks for your reply. I am referring to the use of Airspeed hold in level flight, not whilst climbing or descending, BUT maybe airspeed hold is only designed for use WHEN climbing or descending. I don't know. This is the kind of thing that I am trying to clarify.

[STBYRUD]

Oh sure, that wouldn't make much sense at all. I am guessing (because I am not familiar with the A/P on the F27 specifically) that either the mode isn't selectable in level flight at the selected altitude or it would just climb or descend away from it as the airspeed changes - in any case, it can't do anything to counter an airspeed change without leaving the altitude.

[Dash8driver1312]

F27 Autopilot "Airspeed hold"

DH8D has IAS hold mode in the autopilot. And without an auto throttle it is also only for climbing and descending. If there's no auto throttle fitted, that's what it's there for, level changing at fixed speeds.

[oceancrosser]

F27 and autopilot is not something that was said in the same sentence in my day on the airplane...

[Mach E Avelli]

As mentioned IAS hold without autothrottle is only a climb or descent mode.
The cruise mode is ALT with speed manually controlled by power.
Early versions of the F27 had a very primitive auto pilot. Then along came the Smiths SEP2. Jokingly referred to as "seldom ever performs twice".
In fact, with proper maintenance it was quite capable of holding airspeed in climb or descent, but tended to oscillate a bit trying to be so accurate.
Most pilots drove it up or down on the pitch switch, which had two positions for low and high response.
The Yanks considered that autopilots in turboprops were for pussies. Real men hand flew, so most F27s that went Stateside had them removed.

[fruitloop]

ARR yes the good old SEP2D auto pilot....Better then the AL30J that used to deviate when de-ice was selected on !!! Memories !!

[Amadis of Gaul]

Can't remember the last time I even saw a -27.

[Tinstaafl]

I think you'll find you can select airspeed hold *or* altitude hold, but not both together.

[skyhighfallguy]

alex ford

I have never flown the F27. But I have flown a very nice jet that had airspeed hold.

The autopilot will change pitch in order to maintain selected airspeed. It has no control over the throttles.

A good use for this is climbing at a selected airspeed. At some point you will climb so high that if you selected a rate of climb, the plane would stall.

So, you select a speed and at some point in the climb you would not climb anymore but you wouldn't stall.

While You would NOT select airspeed hold in level flight you could , but the ride would be an oscillation and has no good reason UNLESS you are trying to hold rough air speed and are willing to be off altitude. And in really rough air I would use PITCH hold if you have it.

You would select airspeed hold if both engines quit and you wanted to maintain the best glide speed.


You could close the throttles and descend at a selected airspeed and the rate of descent would vary in order to maintain the speed.

The F27 is an old plane. Think old and you will see why so many old pilots hand flew so much! And they got good at it.

If you lost one engine and had to maintain V2, speed hold might help (I am not sure what your limitations section allows).


So, again. NO throttles. ONLY pitch and that's all she wrote.

OH, on my older plane we also had MACH hold for the times we were in mach land, but I have a feeling the F27 never got close to mach land

[Capn Bloggs]

I am referring to the use of Airspeed hold in level flight, not whilst climbing or descending, BUT maybe airspeed hold is only designed for use WHEN climbing or descending. I don't know.

Alex, you need to do some research on the basics of aircraft operation. You cannot have Airspeed Hold while also maintaining an altitude unless you have an autothrottle. The F27 does not have an autothrottle. IAS Hold is used for climbing and descending (the autopilot using the pitch controls) with a constant power setting. This is basic stuff that would be obvious if one had done an instrument rating.

I very much doubt that it says in the manual that that mode will maintain IAS whilst also maintaining level flight.

[lalbak]

The Fokker 50 has the same IAS mode and it is indeed only used for climbing and descending. With ALT hold activated pushing the IAS or VS mode cancels ALT hold and the A/P reverts to basic pitch mode. The aircraft would most likely leave its altitude to maintain airspeed which results in an altitude alert.

Using IAS mode in the F50 works in smooth air but other than that the aircraft will oscilate chasing the selected airspeed.
We are taught to use IAS mode to maintain vFTO in a single engine situation as it is critical to maintain the speed precisely.

[Dufo]

You don't need to control engine power to fly at same indicated airspeed. Vertical speed is then simply excess or lack of available power.
Good old Saab 340 which I chauffeur has three climb modes, IAS, climb (TAS) and VS, and no autothrottle (ok sort of, but that's another story).

[Dont Hang Up]

Perhaps it is worth remembering that , when one is first taught to fly, it is "pitch for airspeed" and "power for (rate of change of) altitude".

So an IAS autopilot mode is very much back to basics.

[Mach E Avelli]

Ahh, THAT explains why so many pilots struggle with their first jet. It is so long ago I can't remember what my instructors taught, but as they were ex military, I suspect that they said it a little different. I do remember one of them saying that pitch only controlled speed when there was no power, as in a glide approach.
Davies in his bible " Handling the big jets" says that the throttle is the speed control and pitch is the flight path control.
It even works on bugsmashers, so I don't know why it is not taught that way from the start. It would save having to realign one's thinking later.
The basic autopilot is a dumb animal, and certainly if in IAS mode it will vary pitch to hold speed while the rate of climb or descent will be a function of power.
When VS mode is used (if your autopilot is post Stone Ageand actually has this mode) it reverts to power as speed control. This is the mode I prefer for descent in a non autothrottle aircraft as it forces one to keep a hand on the throttles for speed control. But there are so many ways to skin a cat that I am not saying this is the only way to use older tech autopilots.
As someone else has pointed out, with these older autopilots, use of VS in climb is a no no, because the demanded rate of climb could exceed the aircraft's performance capability, with resultant stall.

[skyhighfallguy]

The master of learning to fly, one Wolfgang Langweische in his book, "Stick and Rudder" is of the power to flight path and pitch for speed.


BUT he makes the exception for ultra precise flying.

We teach the idea you power because small planes frequently run out of excess power. Super fighter jets are a bit different as they usually have plenty of power .

BUT ALL PLANES when power is lost can be flown with pitch for airspeed.

AT some point, pilots understand which technique to use when. BUT if you are a novice, pulling back to go up does not always work.

[Capn Bloggs]

Hampster-wheel...

[ACMS]

Mmmmmm

I seem to recall a very long arguement not so long ago about this, it was very long winded thread........

[t7a]

'F27 and autopilot is not something that was said in the same sentence in my day on the airplane...'


How very true! I recall that Hdg Hold was also available - the autopilot would hold any heading it thought might be appropriate but not necessarily the one the pilot wanted!!