Altitude Loss During AP Controlled Flight

[walterleo]

Hi friends:

Thanks for the Christmas present! Have already installed (AM DC8 61H) it. The sound is tremendous! Noticed that from past versions some left: the AP looses altitude while turning at all speeds wheights and flaps settings. Some tinkering around with flighttuning could be desirable?





Many thanks

Walter

[aerofoto - HJG Admin]

We´re aware of this .... as a similar (but also slightly different) issue was discussed on this forum a few months back

tonymadgehjg.proboards.com/thread/8026/dc-pitch-climb-descend-issue

An apparent, but minor, altitude loss during AP controlled flight and in response to course adjustment/s is "quite normal" with these DC-8 simulations .... particularly in the case of the -60´s and -70´s .... but under some (not all) circumstances .... this can also be, the result of a gauge reflex indication too (within the VSI gauge) .... since little evidence of any such altitude loss can be ascertained, in external viewing mode, and again only under some circumstances except in extreme cases and where flight control may also be lost.

When this happens (with an actual and significant altitude loss), then, it generally occurs only during AP controlled flight and within the lower airspeed range whilst manoeuvring for the approach to landing .... particularly in response to acute heading adjustments of around 90* or more. Any such altitude loss should only be minor though, and not at all sustained, and the simulation will/should always promptly recover the lost altitude upon completion any such manoeuvres.

If, during any such manoeuvre, the altitude loss is either substantial or sustained .... then .... the recommended intervention is to apply a little manual trim up input (using keyboard commands .... this will have a positive effect even during AP controlled flight) and which may also then warrant an equal amount of manual trim down input to aid the simulation returning to a level flight attitude. Manual trim response "IS" available (to some extent) in regard to input even during AP controlled flight

This minor altitude loss/recovery has been observed among "all previous version HJG DC-8 FDE´s" and "IS NOT" common/specific, to this years version of these files .... and .... "is not" uncommon to some of our other simulations too.

I don´t regard this particular issue as being serious enough to warrant any further update on our part .... in fact I won´t be messing further with this latest version of DC-8 FDE´S, since I´m "more-or-less" satisfied with them and are unwilling to try´n fix something minor that may well result in breaking something else, and which is "ALWAYS" a risk in regard to manipulating FDE/Model inter-relationships.


Just to recap on all of this ....


1. "Very minor" altitude loss/gain when first engaging the AP ALT HLD function is often evident (nothing more than a brief nod up and down) per VSI gauge indications also, but, this is quite normal and is never prolonged.

2. "Extremely minor" and unsustained altitude loss during course/heading alterations and during high speed AP controlled level flight is quite normal but these simulations "always" promptly auto-recover upon completion of any course/heading adjustment .... if experienced at all.

3. "Minor" but unsustained altitude loss during low speed AP controlled flight (whilst flying any approach to landing pattern) during acute heading adjustments of up to 90*, or more, "CAN" happen .... but .... these simulations should always promptly auto-recover (except in extreme cases) .... or the altitude loss may be restricted though manual/trim intervention.


In the case of # 2 .... flying the approach to landing with a full payload, and with in excess of 20% "total fuel" (weight) remaining, and/or extending the flaps at too high an airspeed will contribute to this minor issue. The flight characteristics of these DC-8 simulations are very sensitive to both power adjustment/s and airspeed .... power needs to be adjusted carefully and the flaps need to be extended at the correct/recommended airspeed.

In the case of flap influence/s .... in order to achieve a desirable/realistic (DC-8 type) approach to landing AOA of between -1* (nose down during the actual approach) to +1* (nose up just prior to landing) in relation to the correct airspeed .... we needed to manipulate certain lift scaler values within the FDE. Many of these DC-8 simulations previously displayed "a ridiculously high" AOA in response to flap deployments, and power/airspeed adjustment/s, during approach to landing, but, as is common in the case of FDE writing any "positive" gain can sometimes result in "negative" gains also .... and that´s just the way FS "IS".

AGAIN .... we know about this particular issue. I (personally) regard it as being "minor". The above recommendation/s will help to avoid/restrict, or assist controlling it .... "IF" and when it´s ever experienced.


Mark C
BOG/CO

[walterleo]

Hi friends:

In my first "flight" today is was more than minor, at 2000 ft 200 kts and flaps 20 it went up and down 1000 ft/ and did recover but lost more than 1000 ft in between. Changing in the flighttunig section of the aircraft.cfg the line

elevator_effectiveness=1,2//1.00

and

elevator_trim_effectiveness=1.2//1.000

means 20 % more effectivness changed the behaviour considerably, so it is really a "very minor issue". I will leave that values in my DC-8-62H.

Kind regards

Walter

[aerofoto - HJG Admin]

200 kts and flaps 20

"THAT" could be a contributing factor Walter.

My recommended descent and approach to landing procedures for the DC-8-62H (and these do vary a bit between DC-8 versions) are as follows ....

The following descent procedures assume maximum payload with no more than approximately 20% total fuel remaining prior to the commenment of descent.

These procedures are intended to ensure descent to FL100 by approximately 30 DME from any destination airport or its nearst NAV aid .... and to promote further descent and entry to any airport approach circuit/pattern, flying downwind, base, and final approach legues prior to landing legues.


"DC-8-62H (high gross weight) & DC-8-63 (low gross weight) (only) DESCENT PROCEDURE"

Descending from FL310 ....

- 110 DME from destination airport .... reduce engine thrust to N2 80% .... maintain level flight to aid airspeed reduction to less than 300 KIAS prior to commencing descent.

- 100 DME from destination airport .... commence 2000 FPM ROD .... reduce engine thrust to N2 70%.

- Passing through FL200 .... reduce engine thrust to N2 60 % engine thrust .... maintain 2000 FPM ROD.

- Passing through FL150 .... reduce ROD to 1500 FPM .... maintain N2 60 % engine thrust.

- Passing through FL1o0 .... reduce ROD to 1000 FPM .... maintain N2 60 % engine thrust.

- FLAP 12 = 200 KTS - adjust engine thrust to control drag induced speed reduction.

- FLAP 18 = 180 KTS - adjust engine thrust to control drag induced speed reduction.

- GEAR DOWN = 180 KTS - adjust engine thrust to control drag induced speed reduction.

- FLAP 23 = 160 KTS - adjust engine thrust to control drag induced speed reduction.

- FLAP 35 = (noise abatement) - 150 KTS - adjust engine thrust to control drag induced speed reduction.

- FLAP 50 = (for emergency use only) - 140 KTS - adjust engine thrust to control drag induced speed reduction.

- APPROACH TO LANDING AIRSPEED (full flaps) = 135 KTS

The above flap/gear airspeed recommendations will ensure an approach to landing pitch attitude which varies between -1, +0, and +1 degrees .... in accordance with the recommended flap/gear extension and airspeeds.

I certainly would not recommend FLAP 20 (FLAP 23 on the -62/63/-72/-73) until 180 KIAS .... or below .... and which also roughly corresonds with GEAR DOWN action as well.

The rest of your experience could (possibly) be determined by a number of variables .... ranging from payload weight and distribuion, fuel weight (not more than 20% for landing is recommended), airspeed, and angle of intercept for the ILS/GS or approach path.

I find the above pocedures do work fine .... for me.


I have compiled similar descent/approach to landing procedure recommendations for all of the HJG DC-8´s and which are located near the end of "SECTION 5:01" of my "DC-8 PANEL INSTALLATION & HANDLING NOTES" manual. These may deviate somewhat from actual real world DC-8 procedures, but, that´s to be expected given limitations imposed by the FS environment.


Mark C
BOG/CO

[Herman]

I have experienced the same altitude excursions as Walter on the DC-8's, of plus and minus 1000ft on the VSI, when, for example, making the turn to the final approach heading for an ILS approach at the recommended speed and flap setting.

The only way that I have been able to minimize this, is by "not" moving the heading bug all in one shot, but rather moving it in small increments to the desired heading.

Herman

[aerofoto - HJG Admin]

I´ve already covered this matter quite adequately .... I feel .... per each of my above responses.

There are 4 details, in particular, which I´ve cautioned in regard to it and which are as follows ....


1. Do not attempt to land with in excess of 20% total fuel remaining .... and this applies to any simulation and not just the DC-8´s.

2. Do not attempt intercept the ILS/GS at angles in excess of 25* degrees.

3. Watch the airspeed throughout the approach to landing.

4. Do not extend either the flaps, or landing gear, in excess of the recommended airspeeds during any approach to landing.


Taking liberties in regard to either of the above "WILL" result in difficulties.


I want to put this matter "to bed" once´n´for all per my following report and re-testing of today .... the folllowing recorded observations of which "are consistent with" flight testing performed using each of these DC-8 simulations (over 3 years) and prior to their re-release during March 2016 ....


DC-8-62H

1. Manouvering into the approach pattern .... AP controlled flight, 4000 FT, 210 KIAS, FLAP 12 selected, 20 degree right bank and no sustained altitude drift ....






2. Manouvering into the approach pattern .... AP controlled flight, 4000 FT, 200 KIAS, FLAP 12 selected, level flight with a +2* degree AI pitch attitude .... a brief/momntary -500 to + 300 FPM altitude drift was experienced during the 20 degree bank and prior returning to a level flight attitude ....






3. Manouvering into base legue .... AP cotrolled flight, 4000 FT, 200+ KIAS, FLAP 12 selected, 20 degree right bank and no sustained altitude drift ....








4. Eastablished on the base legue .... AP cotrolled flight, 4000 FT, 180 KIAS, FLAP 18 selected, level flight with a +1* degree AI pitch attitude .... a brief/momentary -500 to + 300 FPM altitude drift was experienced during the 20 degree bank and prior returning to a level flight attitude ....






5. AP controlled ILS/GS coupled auto-intercept and turn to final approach .... 4000 FT, 180+ KIAS, FLAP 18 selected, level flight .... PLEASE NOTE: THIS IS A VERY ACUTE 90* DEGREE ILS/GS INTERCEPT WHICH IS "NOT RECOMMENDED" BUT WHICH EACH OF THESE DC-8 SIMULATIONS ARE MORE THAN CAPABLE OF SUCCESSFULLY PERFORMING. AN ILS/GS INTERCEPT NOT EXCEEDING 25* DEGREES IS ENCOURAGED AND SHOULD BE CONSIDERED NORMAL IF NOT STANDARD .... once again a brief/momentary -500 FPM altitude drift was experienced (as can be seen below) during this 20 degree left bank and the AP controlled auto-intercept of the ILS/GS and prior to quickly recovering ....





PLEASE NOTE: The very mild ROC/ROD drift which was observed to occur "during acute course alternations" in each of the above cases was per VSI gauge only .... which indicated a slight pitch up/down accordingly (visually confirmed in external FS viweing mode) since no actual altitude loss/gain was sustained as a result of such drift.



6. AP controlled ILS/GS coupled auto-intercept manouvering .... ILS/GS indications already captured, 9 DME from the RWY threshold, passing 3000 FT, 168 KIAS, FLAP 18 selected, with a 700 FPM ROD ....




7. Established on the ILS .... AP controlled ILS/GS coupled auto-approach to landing .... 5 DME from the RWY threshold, 158 KTS, FLAP 23 selected, 1000 FPM ROD, and passing 1000 FT .... THIS IS THE ONLY STAGE OF ANY APPROACH TO LANDING THAT I CAN OBSERVE AN INDICATED 1000 FPM ROD .... the AI pitch attitude ranging between a -1* and +1* degree indication as the simultion "attempts to" maintain the G/S indication .... PLEASE NOTE: THESE SIMULATIONS ARE NOT EXPECTED TO BE ABLE TO HUG ILS/GS ALL THE WAY DOWN TO THE RWY SINCE THE DC-8 DID NOT REPRESENT AIRBUS TYPE TECHNOLOGY. THESE AIRCRAFT ARE REPRESENTATIVE OF A TIME WHEN ILS/GS FLYING WAS IN ITS INFANCY. THEREFORE THE PANEL/GAUGE ILs/GS INDICATIONS SHOULD BE REGARDED AS "A GUIDE ONLY" .... WITH POWER, AIRSPEED, PITCH/ROD ALL NEEDING TO BE CONSTANTLY MONITORED, AND ADJUSTED, THROUGHOUT THE APPROACH TO LANDING IN ORDER TO AVOID OVER OR UNDERFLYING THE ILS/GS. FOR THIS REASON ALL APPROACHES TO LANDING USING THESE DC-8 SIMULATIONS ARE BEST FLOWN "MANUALLY" (recommended) .... AS WAS PREDOMINANTLY THE CASE WITH THE REAL WORLD AIRCRAFT. THE DC-8 WAS REALLY A "HANDS ON" AIRCRAFT IN MOST IF NOT ALL RESPECTS ....






8. Established on the ILS .... AP controlled ILS/GS coupled auto-approach to landing .... 3 DME from the RWY threshold, 138 KTS, FLAP 50/FULL FLAPS selected (PLEASE NOTE: ON DC-8 AIRCRAFT FLAP 50 WAS NORMALLY SELECTED ONLY IN EMERGENCY SITUATIONS .... OTHERWISE FLAP 35 WAS THE MAXIMUM FLAP SEETTING USED FOR FOR ANY NORMAL APPROACH TO LANDING AND NOISE ABATEMENT), less than 1000 FPM ROD "and further reducing" in conjunction with the ILS/GS indications, +1* degree AI pitch indication, and passing 500 FT prior to landing ....






9. Landing .... 133 KTS, AP disengaged, manual flare commenced from approximately 20FT, all 4 engines spooled back, and the AI Pitch attitude not exceeding +2* to +3* degrees ....








So long as these DC-8 simulations are each flown/handled properly, then, no difficulties should result.

Following the recommended procedures is imperative though in regard to realising a successful/enjoyable DC-8 experience .... in FS .... and when using any of these simulations.

As the above report should demonstrate .... I "CANNOT" replicate any major issues .... at all .... at least not to the extent of what´s been discussed and to which I´ve replied to earlier within this particular thread .... and that´s despite using precisely the same FDE files everyone else has and even (purposely) performing some aspects of these the above testing a little more extremly than is recommended within any of my guides .... just to see what would develop as a result.

I´m "NOT" saying there´s no credibility to other peoples experiences/reports since we know only too well, from past experience, that due to any number of vasriables unique issues "CAN" and "DO" sometimes arise .... and whach "ARE" extremly difficult to understand let alone troubleshoot.

All I´m saying .... per this report .... is I (personally) arent able to generate, and nor am I experiencing, anything of major concern´to me .... and which "DOES" cause me to become dubious (though "not in the case of Walter) in regard to some peoples interpretation and application of the procedures we recommend.

Mark C
BOG/CO

[Herman]

Mark, Please note. My comments were not meant to irritate you or the HJG DC-8 Models.

I was only intending to reflect that my experiences were similar to Walters, not with the the DC-8-62H but more so with the earlier DC-8's.

However, I will again, do some test flying with a DC-8-40 Model to observe its behaviour.

Herman

[Herman]

Just completed a test flight using a DC-8-40 Model and using the landing and approach profile recommended. My results were very similar to what Mark had demonstrated.

The previous results that I had mentioned and recalled, I think, were prior to the latest FDE and CFG modifications when the default auto pilot turns were set at 25 degrees. The current turns are set at 20 degrees and I feel that that may responsible for reducing the VS excursions.

Herman

[aerofoto - HJG Admin]

Please note. My comments were not meant to irritate

Not "irritated" at all

We take all reports "very seriously" here .... "never" personally though .... and regardless whether these be negative or otherwise.

And despite the time often required in order to respond with desirable clarity .... 9 out 10 times it´s a pleasure to try´n do so

The previous results that I had mentioned and recalled, I think, were prior to the latest FDE and CFG modifications when the default auto pilot turns were set at 25 degrees. The current turns are set at 20 degrees and I feel that that may responsible for reducing the VS excursions.

It "DOES" help .... BUT "TECHNICALLY SPEAKING" .... the AP authority, in regard to its control over Bank Angle, should be set at "15* degrees" .... and which may improve things even more.

This can still be done in these DC-8 simulations .... if folk wish to do so .... BUT IN DOING SO (applying an AP related MAX BANK = 15 edit) .... the following will result ....

1. An increased turning arc during AP controlled flight in response to HDG related inputs .... and this turning arc will become greater still in proportion to increased airspeed.

2. AP controlled VOR NAV intercepts .... and ILS/GS coupled intercepts for the approach to landing also .... will both become less decisve or prompt with any incidence of weaving during either intercept being a little more prolonged

In the case of ILS/GS intercepts i particular .... and as I´ve previously stated within this particular thread .... approaches to landing (flying the Dc-8´s) should be flown "MANUALLY" .... right from the initial intercept and all the way down to the RWY, but, which does require careful engine thrust, airspeed, and pitch manipulations (in order to avoid over/under flying the ILS/GS indications) and despite the fact that these simulation can fly AP controlled ILS/GS couple approaches quite reliably and down to around 200 FT prior to actully landing .... as rlated per my earlier reply postings within this particular thread.

Mark C
BOG/CO

[walterleo]

Hi Mark:

Speaking jokingly: The HJG FAA "placards" the AP: ILS must be flown manually.

From my experience the AP holds the ILS GP well down to around 400 ft flown at 140 kts it will loose than the GP.

My problem is that: Not the Vertical speed indications while turning on AP but the altitude loss of more than 50 ft. 50 ft altitude loss would be limit for an FAA testflight for ATPL pilots while flying manually. My idea was, that an AP should do it it least the same. Yesterday with my amended aircraft.cfg it could do that. Did not crosscheck it against the original cfg. I am not complaining about altitude excursions during configuration or speed descent or climb changes.

Kind regards

Walter

[aerofoto - HJG Admin]

They actually do it quite well all the way down to around 200FT .... provided the total remaining fuel load is less than 20% (more than that and the simulation becomes heavier and which is likely to change their entire approach to landing ILS/GS profile) .... BUT .... if using my own recommended procedure/s then 132/133 to 135 to 137/138 KIAS ("depending on DC-8 aircraft type" since these each differ) with full flap is far better .... just a small variation od 1 to 2 KTS over/under the recommended approach to landing airspoeeds for each Dc-8 type "DOES" make a differnce despite it not seeming much.

HOWEVER .... in the real world .... DC-8 approaches to landing were mostly flown manually .... "guided by ILS/GS indications" (and the same applied also to the B707/720, CV-880, and CV-990 too) .... according to many retired aircrews with whom I´ve communicated along with many contemprary reports/memoirs I´ve read as well.

"IN FS" .... these simulatios will fly such approaches to landing (and even VOR NAV too) better than they actually did in the real world .... remembering these aircraft represent very early 1950´s/60´s era technology rather than auto-technology of the 1970´s/80´s and as cwas installed on aircraft like the A300B, B747-100/200, DC-10, and L-1011 .... which could (under most circumstances) auto-fly the the appoach to landing .... to the extent of even auto-flaring (L-1011) and then actually landing the aircraft too.

Practicing flying appoaches "MANUALLY" .... forcing one to "HAVE "TO" constantly adjust pitch, power, and airspeed in order to fly the approach in conjunction with the displayed ILS/GS indications .... is challenging, but, can really teach one "A LOT", as well as boosting ones skills too .... not to mention giving one a real taste of what these old first generation narrow-body jetliners might have been like to fly. It´s "NOT" easy (at first) .... but like anything else .... it´s something that becomes easier (later) with much practice and experience

Welcome to the "ARC"

Mark C
BOG/CO