Performance Data at Indicated Airspeed

[Tail Bloater]

Over the past 18 months I have been conducting a personal investigation of the relationship of CLIMB Performance Data with regard to Indicated Airspeed (IAS). I have discovered that on the particular type of helicopter that I conducted the tests there is an error in the Performance Charts. The charts relate to IAS in the climb at Vy (65KIAS up to 5000 ft Hp). However,in the climb the ASI under-reads by an average of 10Kts. Therefore the actual airspeed flown at 65KIAS is in fact 75 Kts. This is position error (PE) and no mention is made in the flight manual.
Question to you is:-

Do I stick with the listed performance data?

[Nick Lappos]

Generally, any charts that are part of an approved Rotorcraft Flight Manual have all that IAS position error magic built in. The FAA and CAA are meticulous about the accuracy of procedural data as presented to the flight crew. For the S-76 and S-92 flight manuals, we must use TIIAS as the published data for Cat A procedures and climb data (TIIAS = True Instrument Indicated Airspeed, which is what would be read on an airspeed gauge with no internal error ).

Do you know that the Vy is actually 65 knots? Does your climb data support the possibility of error?

[Tail Bloater]

How I was first alerted to a discrepancy was during an autorotation. The IAS was suggesting a forward speed of about 35 KIAS, but the machine was acting as though it was in a zero airspeed configuration. I could only assume at the time that there was an error with the ASI or if that was correct that the position of the pitot tube was such that Position Error was apparent.
Over the past 18 months when the climatic conditions were favourable (similar temperature and zero winds) I compared climbs at Vy (quoted in the flight manual as 65KIAS at the height band I conducted the tests), and in every instance at a steady IAS of 65Kts the Ground Speed readout on the GPS showed an increase in forward speed. I have done 40 - 50 climbs on different headings and compared GS to IAS and the results were similar.
Inbetween each climb I performed various steady autorotations and again compared GS to IAS and the results confirmed that there was an error here as well.
I fully appreciate that the responsible authorities will want hard evidence that performance data is correct that is to be included in the flight manual.
What I am trying to discover is (a) should I fly at IAS at Vy, (b) during autorotations never fly at less than 70 Kts, (c) notify the manufacturer that I have doubts about their data.
Point of note: 'Where do the authorities get their performance data from to ensure that it is correct'. The answer I'm sure is fron the manufacturers flight test data.

[idle stop]

You are right that performance data will come from the manufacturer's flight test data. However, it is usually spot-checked (at the very least) by the certification authority during type certification. Each individual production series aircraft must meet the spec performance for its initial and subsequent C of A airtests.
Airspeed indication errors may be due to a variety of causes. The obvious starting point is the indicator itself. Or is there a problem with the pitot (dynamic pressure) sensing probe alignment? (Possible blanking.) How easy is balance keeping? An aircraft with poor out-of-balance tactile cues could result in the airflow into the pitot head being reduced by inherent yaw. A static blockage or partial blockage might affect things, but you might also see effects on the other pressure instruments.
Commonly, hanging bits on aircraft may upset the airflow past the static ports, so this is often investigated when modifications are being approved. Flights over an accurate course in opposite directions, using raw timing, GPS, kine-theodolites, a suitably calibrated 'chase' aircraft or a combination of these methods is used to extract data to compile or update pressure-error correction charts. (This should be legislated as part of the modification submission and the design organisation would, in UK, have to have prior approval from the CAA flight department for the pilot actually flying the test.)
There are some answers.
If you have serious doubts as to the accuracy of indicated airspeed you should bring this to the attention of your maintenance organisation.
Now would you like to indicate the aircraft type, please?

[212man]

What you have not stated is how the rate of climb compares either with the graphs, or with varying IAS. Surely, the point is that the R of C is a function of the TAS and that the IAS slected will reflect this. Hence the fact that the IAS will should normally decrease by about 1 kt per 1000 ft in the climb (about 1% per 600 ft actually, but given the average Vys around and the need to be practical, 1 kt per 1000 ft is how it's written).

You state the relationship with GPS g/s, had you calculated the TAS under the conditions? Was there really no wind affect?

In general, it is true to say a manufacturere will want to give the best possible figures for an a/c, and the regulator will make quite sure they are not exagerating them before certifying the FLM.

[Shawn Coyle]

Have you had your pitot system checked? Sounds like it could be a maintenance issue.
What type of helicopter?
Shawn

[Tail Bloater]

For Shawn Coyle, idle stop and 212man.

The airspeed indicator has been checked, it was my first thought, and it checks out 'satis' and this is accepted as in forward, straight and level flight the IAS = Ground Speed as far as I can assertain in zero wind conditions.
The performance graphs refer to Vy and therefore accordingly I have flown Vy at 65KIAS as required by the text.
I have sent the data that I collected to the manufacturer and await their reply.
I believe that there is a position error correction which needs to be applied, or if this is not the case and IAS used for climb performance there is a possibility that at higher altitudes this may ulitmately effect the expected performance.
If I were, for instance making a pre-flight calculation for climb performance from 1000 ft to 5000ft Hp , the graph would give a RoC for a set temperature and Mass (Weight). If this performance were unobtainable would it be that the engine was under-performing. If the performance was achieved would this mean that IAS was correct or that better than normal engine performance was the result.
If you would please wait until I get a response from the manufacturer before I name the model.

Balanced flight is easily maintained and there are positive airspeed changes when not in balanced flight, as one would expect.

My next tests will involve flying the descent and climb out of balance by 10 degrees left and again right, to see if there is an effect there.

Thank you all for the clues and ideas

[GLSNightPilot]

I'm afraid attempting to correlate GPS groundspeed with IAS is illusory, at best. Just because there is little or no wind on the surface, don't expect no wind at a few hundred feet altitude. There is almost always some wind at altitude, & I think believing in 'no wind' conditions will in fact cause you trouble, in more ways than one. The core question is what is the rate of climb at different indicated airspeeds? Groundspeed must be disregarded, the only things that matter are indicated airspeed vs ROC. Indicated airspeed is not the same as true airspeed, nor calibrated airspeed. The only thing you have presented in the cockpit is indicated airspeed, & that's what the charts should be based on.

[Shawn Coyle]

You might check for leaks in the static system, or someone may have snuck a 'snubber' in the line, which can cause problems. We had a problem with the pitot static calibrations in one helicopter we were verifying when I was with Transport Canada - the pilot and copilot airspeed indicator had different corrections in climb and descent. Turns out one of the systems had a snubber installed and one didn't. The snubber is just a restricting device to slow down rate of change of static information. - You won't see any effect in straight and level flight....
Might be worth a look..

[Arm out the window]

Also you get a couple of extra knots of TAS (and therefore groundspeed in nil wind conditions) for every thousand feet of climb at a given IAS, so you would expect an increase in GPS calculated groundspeed in a climb, and the reverse in a descent.

[Genghis the Engineer]

ASI Calibration

I'm not so familiar with RW codes, but in any FW code, more than 5kn error wouldn't be acceptable. Having said that, 10kn error doesn't seem that unreasonable, particularly given what a horrible system a helicopter is to try and get sensible pitot and static readings in.

To calibrate IAS against GPS, you need to initially fly a series of speeds into wind (having determined exact wind heading, which I do by flying a range of headings at constant IAS until I get the lowest GPS groundspeed), noting the GPS GS at each speed. The do the same downwind. If there was a rate of descent you need to note that by timing through two fixed heights as well.

With any trig correction for rate of descent, the TAS is the mean of the two groundspeeds (into wind and downwind), and then it's only necessary to correct for density altitude to give CAS.


There's a data reduction sheet that you can use at http://www.bmaa.org/AW_043.pdf


Best Climb Speed

It's not always the case that the actual best climb speed is what's given in a POH. It may be that other issue (handling, cooling, climb angle ?) were factored in for reasons known to the test team but not reported in the final manuals.

Such speeds are always given as IAS, although any CofA aircraft should have a PEC curve tucked away in the POH somewhere - irrespective of whether it's fixed or rotary. If you haven't got one, I'd suggest you're missing some aircraft data that you should have.

Climb .v. level or descending flight

The nature of a helicopter is that you will almost certainly get different PEC in cruise, climbing and descending flight. I'd suggest that PEC in a helicopter probably need establishing in each mode and plotting separately. If you are only interested in Vy, it should be possible to do the ASI calibration as above in the climb, and apply climb correction identically to the descent correction.

G

[Tail Bloater]

Thank you GLSNightPilot and Genghis for your comments.

I have done 'calibrating flights' by flying on different headings and noting the differences and making due allowances for any observed wind effect. I am aware that this is rather hit and miss but all my results however constructed seem to point in one direction that PEC is relevant in descent and climb.

Shawn I take your point about the 'snubber' as there is a tendancy for the ASI to 'hold' onto airspeed when quick stops are performed. It is possible to come to the hover with 35-40 KIAS showing and there is a delay of 1-2 seconds before the ASI reads the expected reading accounting for local wind effects etc.

Is PEC given for any type of helicopter when climbing/descending?

[Genghis the Engineer]

Fascinating. I just looked up JAR-27 which is the certification code for helicopters up to 7,000 lbs. There is no requirement for them to give you PEC data or any climb data beyond that pertinent to either take-off performance, or determining the height/speed ceiling. It is a requirement that your ASI should be within 3%/5kn at any forward speed above 80% of climb-out speed. It's apparently not a requirement to compare this for climbing and descending flight - the implication (if not specific statement) of the standard at 27.1323 is that it only needs assessing in level flight.

As a flight test specialist it staggers me that they are allowed to give you so little performance data. As a certification Engineer, I'm pretty certain that they'll give you no more than the minimum that they have to.

G

[Shawn Coyle]

Genghis-
shocking, isn't it.
It's even more interesting when you consider that the FAA allows helicopters to use a 100' DH on a normal ILS. No position error corrections, etc.
Why we put up with this?
Shawn

[GLSNightPilot]

the FAA allows helicopters to use a 100' DH on a normal ILS.


Huh? I've never seen this on an ILS. DH is usually 200'. You may descend to 100' above the touchdown zone elevation if you have the approach lights in sight, but this is not the decision height, you are then operating below the DH with some surface features in sight. Technically not the same thing.

[Genghis the Engineer]

Why do we put up with this? Because the accountants will stop paying us if we make a habit of exceeding minimum requirements, it's just the same on FW aircraft, and not very nice to contemplate in either.

Maybe I should stop getting so worried about all this nasty real-world stuff and just go and teach instead. Any vacancies for a FW-FTE over there Shawn?

G

[212man]

Genghis,
I doubt if you'd enjoy it over there; think of the incessant sunburn and all those Californian babes!

[Genghis the Engineer]

It might be worth that hardship not to have to deal with UK CAA any more.

G

[Shawn Coyle]

Genghis
send resume
[email protected]

[Genghis the Engineer]

Hmm, lets see. For 2 weeks here it's not gone more than 6° above freezing, nor have we had any relief from constant IMC.

Wilco.

G

[Tail Bloater]

Dear Idlestop and Shawn Coyle.
And to everyone also.
The answer to the 'problem' is that there was/is a blockage in the Static system at the point before the static pipework splits to the Altimeter and ASI. Changing to the alternative source had no effect. This provides the error which manifests itself as an incorrect indicated airspeed and lag on thr altimeter due to the incorrect static pressure delivered. Hence the over-reading of airspeed during descent and under-reading during the climb. This isn't too greater a problem when flying VFR but the dangers are there if flying on instruments. However it would be most helpful if PEC correction data were available in cercumstances other than S&L. During my time flying Gazelles IFR and on approaches there was the Aircraft Allowance to put into the computation for IFR arrivals to make allowances for altimeter lag and pitot position errors at that angle of approach. It is always worth checking the relationship of data given by the ASI and that by the GPS. There should be the W/V in there somewhere.
Unless somebody knows differently.

[Shawn Coyle]

Glad to be of assistance in troubleshooting the problem.
You are right about the lack of information on pitot static errors. The FARs take care of it, sort of, by wording that requires minimal errors, unlike the FW which requires even the minimal errors to be put in a graph in the Flight Manual.
By the way, only the UK military uses Aircraft Allowance - everyone else works on the assumption that the manufacturer had better make a pitot static system that has such small errors that you can take the aircraft to the published Decision Height without worrying about hitting the ground.
But that could lead to a whole discussion about QFE and other things...

[Genghis the Engineer]

Was it me or did I almost, almost, hear a North American suggest that QFE might be useful there?

G

[idle stop]

TailBloater: Glad to have been of assistance. Happy Christmas.

[Shawn Coyle]

Genghis
QFE is something that only works well in the UK - no airfields higher than 400' AMSL.
It wouldn't work nearly anywhere else - Leadville Colorado is 9,975' AMSL - so you'd be at an altimeter setting of somewhere around 20" of mercury if my math is right. They don't make altimeters with that much adjustment. And even then it's only good for the immediate airport vicinity.
In the long run, we'll be using GPS for altitude - it's already as accurate as any baro system, requires no transmission or read back of altimeter settings, introduces no significant errors after the spheroid is corrected, etc.
And when we look back on using barometric pressure for height measurement we'll be saying - who ever dreamed that up?

[idle stop]

To answer Shawn, and off thread, it was surely the engineering student given an aneroid barometer and asked to measure the height of a tower block. Came up with about six solutions, bar (no pun intended) the obvious!

[Genghis the Engineer]

I once spent 2 weeks on an aircraft carrier. After a week of morning met-briefs, I finally worked out by the Royal Navy didn't give QFE. I suppose I was young and innocent then.

I'm perfectly acquainted with the QFE/QNH debate, it's one of those old chestnuts that'll never go away - but causes mild amusement with visiting North Americans. But concerning barometric pressure - what else could you use? Radalt is too expensive for simple aircraft and doesn't work at altitude or whilst manoeuvring, temperature is too coarse and varies too much at S/L. Just as Churchill said about democracy, pressure altitude is perfectly awful, but better than all the alternatives.

I suppose now we have the geometric altitude given to us by GPS, but even that we'd have to be careful about since the world just aint that round, and the update rate on GPS isn't really up to aircraft manoeuvring anyway.

G


(N.B. Idle stop, I think the correct answer is go to the Janitor and say "if I give you this nice barometer, will you tell me how tall the building is").