Positive Rate
 

These quotes below(of which I have removed some info I felt to be non-pertinent were made on another thread.

Subsequently I found a note mine for a particular modern aircraft that says.....

VSI indication comes from the IRS, therefore when the nose is raised, there will be a positive climb indication even though the main gear is still on the runway.

So would most agree that the VSI is not a good instrument alone to use a determining a positive rate of climb for gear retraction after rotation and that the positive rate indication is initially due to the IRS moving upward.

You NEVER rely on a single instrument! For the Positive rate" call, you use (as a minimum) VSI, altimeter, airspeed, pitch angle, and engine thrust, all AFTER the aural sounds of the relays clicking from the ground sensors.

Nah, I just look out the front!

Wouldn't the old simple kind, be subject to the same pitfall since the static ports are in the nose area? I've never looked until after we're airborne, so I don't know but I think it stands to reason.

I often see an impatience by my colleague of the day about calling "positive climb". And i always wonder why. Normally all engines are working correctly, in which case there is absolutely no hurry to raise the gear at the first twitch of the VSI. And even with the often trained V1 cut it is better to assure that you are in a stable climb before raising the gear as it could be an even worse day if you do it on the first twitch and then some flying inaccuracy leads to a ground contact.

I was trained, quite long ago, to make sure by at least three independent sources that we are indeed positively climbing away from terra firma. Which seems to be a quite good idea in my mind.

"I often see an impatience by my colleague of the day about calling "positive climb"."

I wonder if those colleagues belong to this group?

“Positive Climb”

Thanks,

Appreciate the replies. Hoping for a little more technical input about the IRS.

The IRS position, relative to the nominal centre of gravity position, is known. The IRS can determine it's own movement, both in terms of translation and rotation. So it would be entirely possible to correct IRS data to any other position on the aircraft - and this is done in some experimental aircraft instrumentation setups. The error from having sensors very far offset from the centre of mass can matter. Though conversely, since the pilot is also not at the centre of mass, and it can be a pilot sensation you are interested in, sometimes you want the sensors offset.

So it is true that the raw IRS data is affected by the position in the aircraft, but that doesn't necessarily apply to any processed data.

@Vessbot - yes, the pitot/static system is going to be affected but the effect here is less that the static ports are moving relative to the ground and more that the aerodynamics around the nose is going to be changing as the aircraft rotates, and this effect is very much dependent on exactly where you are in the ground effect, and won't be exactly the same as the PEs you might see for similar angles of attack away from the ground. Obviously all depending on the specific design.

The altimeter should be used to determine a positive climb as well as a IVSI. Look what happened with the EK crash a few years back. They had a positive climb initially then hit the deck a few seconds after when the gear was in transit. The altimeter would have moved only 50ft.

Airbus FCOM or FCTM do not give any guidance about where to check +ve climb.

Yes, i was surprised when i discovered that. Of course, when reading this discussion i checked both the FCTM and the FCOM and found: nothing. Every little thing is defined, but not what positive climb exactly is.

And i just checked my old 737 FCOM (which is a pretty old version as i transitioned 6 years ago to the A320) and that is what it has to say:

"Do not retract the landing gear until a positive climb is indicated on the altimeter, preferably radio altimeter. Because of ground effect the altimeter may not show a positive climb until the airplane is 35 to 50 ft above the runway. Undue haste in retracting the gear is neither necessary nor desirable."

Especially the last sentence is in my view quite important. Otherwise, the result shown above might happen. Or a Dash-8 on its belly in SCN because the FO retracted the gear too early.

I flew with somebody a while back who obviously thought I waited too long to call “positive rate”. When he was PM, he’d make the call when we’d barley broken ground. Eventually, he’d start to call positive rate on his legs. It didn’t make me reach for the gear handle any sooner though. I never figured out what the hurry was.

https://cimg7.ibsrv.net/gimg/pprune....6ffb824b05.jpg
The only guidance for +ve climb is this training DVD. It clearly says "When V/S positive and RA has increased". So not much theory in Airbus. In any case better be late than early.

First large type, written in SOPs: Announce "postive xyz" after VSI indicates 500' fpm and RA is increasing. Smart people.

Simple and effective. Having read this and the previous thread, did not see any meaningful difference in other suggested techniques. Or any time on the line. Fits the Airbus snapshot too.

Little bit more info on the ADIRS that I can share.

The ADIRS calculates Inertial Vertical Speed and Baro Inertial Altitude. This Inertial Vertical Speed is what is shown on your PFD. It is not compensated for placement offsets wrt the C of G, a sudden rotation of the aircraft around the y axis (pitch up maneuver) will show as non zero V/S on the PFD.
The Baro Inertial Altitude is basically barometric altitude from the static ports augmented with the Inertial Vertical Speed. The latter is important since the V/S needs to be reflected on the Altitude Tape and vice versa. Also, integrating Inertial Vertical Speed to augment Barometric Altitude allows for a very quick and responsive altitude tape. Barometric sensors have considerable lag due to heavy filtering and their mechanics.

The filter loop is designed such that the altitude tape will always converge to the barometric altitude. Integration errors (due to accelerometer imperfections) will not result in a diverging altitude over long flight (obviously).

Always best to make sure the whole aircraft is positively climbing and not just the static ports.

Quite so.

It is really not very difficult to check that your IVSI is positive and your Altimeter and your Rad Alt are both increasing before calling "positive climb".

Indeed. And there is absolutely no rush at all.

Keeping it simple and safe.

I know I use 3 things before opening my mouth to say positive rate and it all occurs in a couple of seconds.

SENSE: if I can hear T/O thrust and I can see the attitude passing through 12* I know that we are probably airborne.
and
INDICATION: check IVSI
and
CUE: my peripheral vision confirms for me the ground is getting further away.

”POSITIVE RATE”

T/O thrust should have been not heard but seen and confirmed as N1 or EPR a long time ago. And it's not a matter of probability but a confirmation which comes from RA, normal altimeter and VS not from attitude.