B707-420 altitude loss during turns

[alemaobaiano]

First thanks for all the hard work making these wonderful aircraft available for FREE ;D

I've recently started flying the -420 series (v7) and overall I'm delighted with it....except for losing altitude during low level turns, typically when flying a procedure as part of an instrument approach with autopilot heading mode selected. It's probably me doing something wrong, so can any experience 420 pilot let me in on the secret?

Last night I flew an approach, Vref was given as 143kts, so I lowered the first stage of flap and flew at 168kts (Vref +25), 3000' outbound and turning inbound with Alt Hold and Heading modes selected. During the turn inbound I lost 1000' before I took over and flew the rest of the approach by hand. That's not a big problem, but sometimes I do want to fly approaches with autopilot down to DH.

Any ideas?

Thanks

[jimhalinda]

I find the 707 autopilot somewhat sluggish compared to the DC-8, for example. So the 707 definitely has a harder time keeping altitude in a turn.

Also, perhaps it is because of the greater wing sweep of the 707 that it loses more lift in a turn? Just guessing.

Finally, how much fuel were you carrying? At heavier weights the problem would most likely be worse.

Just guesses on my part, I'm not a dynamics expert. One of the HJG folks will probably give a better response.

Regards,

Jim

[aerofoto - HJG Admin]

Loosing a little altitude in turns is not an altogether unusual aircraft response in FS .... but .... I don't think I've seen anything like 1,000 FT being lost/traded in any turn .... or do you mean an indicated 1,000 FPM ROD being induced during turns.

Either way .... I can't say I've seen anything that acute !

Something Jim's replied is precisely what I was thinking though .... in that "IF" you're too "HEAVY" .... then FS aircraft response like that which you're reporting may possibly then be observed .... and probably isn't too far out of order.

Power settings in relation to aircraft weights are probably quite critical in respect of these old B707/720/C135 aircraft .... and other early types too.

Personally .... I wouldn't ever try landing with more than a 25% total fuel load .... and even that's probably a little too heavy for comfort and flight profile as well.

Mark C
AKL/NZ

[alemaobaiano]

Thanks for the replies gents

Fuel was just over 12000lbs spread over the 4 "engine" tanks, standard pax load as per the config, so nothing outrageous.

Mark, it was a loss of 1000' altitude during a standard turn with A/P. Manually it's quite easy to hold speed and altitude in the same turn just on the trim (I flew the same approach later to see), so it's not a big issue, except that it was more pronounced than other classic types I like to use. Turns at higher speeds and altitudes are normal, it just happens during approach. All the more reason to hand fly approaches ;D

Thanks
Tony

[aerofoto - HJG Admin]

when flying a procedure as part of an instrument approach with autopilot heading mode selected

I flew an approach, Vref was given as 143kts, so I lowered the first stage of flap and flew at 168kts (Vref +25), 3000' outbound and turning inbound with Alt Hold and Heading modes selected. During the turn inbound I lost 1000' before I took over and flew the rest of the approach by hand.

Turns at higher speeds and altitudes are normal, it just happens during approach.

In AP controlled flight mode .... and when coming onto or near any ILS beam .... and especially if one was already banked when the APPROACH MODE was activated .... then I suppose such a loss of altitude could occur.

Since this apparently isn't so acute during other manual or AP controlled flight modes then it may be an FS anomaly in relation to ILS coupled approaches.

I'm aware of a couple of other similar ILS/AP controlled flight mode reports in and in respect of other FS aircraft. ILS coupled AP controlled approaches in FS sometimes aren't perfect .... and I've even experienced some variation between different FS airport RWY's too.

Interesting feedback though !

Mark C
AKL/NZ

[bluestar]

Tony,

These HJG tends to run out of trim in turns when using the autopilot, especially when a little slow on approach (the deck angle +5.0). It's just the nature of the beast. But it does well when hand flown.

Some of the first/second generation autopilots in jets left something to be desired. It was not unusual to disconnect the autopilot in turbulence because the pilot could do a much better job of holding the altitude. In Jurassic jets the pilots hand flew most of the approaches. In fact SWA tends to hand fly most of their approaches (HUD).

Bill

[alemaobaiano]

Thanks again folks

Now that I'm officially curious I spent most of my evening flying the same approach with different configurations.

Hand flying at Vref+25, flaps 14 is easy, trim around +4-5, height loss is minimal.

Same situation with A/P and height loss is of the order of 1000'-1200' during a 180 turn, trim stays in the green but at the lower limit. RoD hits 2100 down before the A/P starts to respond.

With A/P, Vref+30, flaps 25, height loss is better at around 600', RoD peak only 1500 down, trim stays below +6.

With A/P, Vref+40, flaps 14, height loss acceptable at 300', RoD less than 1000 (peak), trim barely moves.

I did try the last example with flaps 25 but there was little discernible difference from flaps 14.

So all I need to do is modify my approach procedures with A/P or hand fly if Vref+40 would take me over ATC speeds restrictions (170kts at some airports I use).

Thanks for the help
Tony

BTW Mark, I have found ILS capture to be very good with this model, better than some commercial products.

[Herman]

In addition to all of the above, I just wanted to add my experiences regarding AP engaged turns on most HJG aircraft. I have found, that by moving the heading reference knob slowly ( one mouse click at a time, rather then a rapid positioning of the heading indicator) I can achive very smooth turns with a minimal change in aircraft altitude. On ILS approaches it just requires starting the turn a little sooner when intercepting the localizer.

Herman

[Mike Monce - HJG]

I can confirm that Herman's procedure is the way to go. The reason being that the bank angle is not very limited so by moving the heading bug fast the planes goes to large bank angles and results in a uncoordinated turn.

This can be changed if you wish: in the cfg autopilot section find the line max_bank . Changing it to 15 would limit the bank angle to a more normal turn. We have never really set this value beyond the default but could easily do so on future releases if people would like that changed.

Mike

[Herman]

Just a bit of clarrification regarding "one mouse click at a time".

This only applies, until the aircraft is in its normal bank angle. After that the heading reference can be placed to the desired heading, otherwise, as you could imagine, if you made a 90deg. turn it would be pretty annoying to click the mouse 90 times.

Herman

[alemaobaiano]

Thanks folks

I have noticed that using the A/P turn knob the height loss is much less, even at the same final rate of turn, so perhaps the A/P is a little "enthusiastic" when entering a banked turn.

I'll try Mike's idea about the bank angle limiter, but I've just got in the habit of hand flying from the IAF, much more stable and more fun.

Thanks

[Harerton Dourado - HJG]

The keypoint is: treat these old ladies gently!

[aerofoto - HJG Admin]

treat these old ladies gently!

YYYYYYYYYYYYYYYYYYYYYYEP .... be good to them .... and they'll be good to you too ! ;D

Mark C
AKL/NZ