Tag Archives: Wind Correction Angle

Watch this huge C-5 Galaxy doing an “almost” touch and go at Westover Air Reserve Base

Filmed at Westover Air Reserve Base, Massachusetts, on Sept. 20, 2012, the following video shows a C-5B Galaxy aircraft of the 439th AW (Airlift Wing) doing an “almost” touch and go during a training sortie.

Noteworthy, on final approach, due to the crosswind, the pilot “crabs” the C-5 to apply a WCA, Wind Correction Angle (nose and tail are aligned with the wind direction while the aircraft follows a different course).

Just before touchdown, the pilot reduces the WCA angle (he “decrabs” the plane) to align the wheels with the runway but, in this case, the aircraft doesn’t touch the ground and performs a go around.

Wind correction angle

I recently explained how approaches to RWY 13 in Hong Kong Kai Tak airport could be spectacular, thrilling and somehow dangerous, especially with crosswinds.

I’ve often discussed crosswind (xwind) approaches (type “crab” in the search box to find the related posts) and explained that a common procedure used with xwind components, the aircraft is “crabbed” (meaning that a WCA, Wind Correction Angle, is applied by aligning nose and tail with the wind direction while the aircraft is following a different course).

Just before touchdown, the pilot usually reduces the WCA angle in order to prevent landing gear damages; however the “decrab” is extremely important on dry runways, because of the side load that could stress the gear on the gear, while it is not that important on wet runways.

Decrab is the phase of the landing with xwind in which the airplane is vulnerable to the gustings and for this reason, all aircraft have to apply cross-controls: left rudder, right aileron (if wind is coming from starboard) meaning rudder and aileron in opposite directions.

This doesn’t mean that the aircraft has always to lower the wing on the upwind side, but this may be required to keep the aircraft on the runway even though many experienced pilots landing on dry runway are able to land with levelled wings.

Crab and decrab must be applied by all kind of aircraft. People often thinks that only civilian wide bodies have to apply a WCA but also small fighter plane have to aling the nose with the wind direction as the following images, taken by Giuseppe Ferraro, show.

Giuseppe visited Trapani in July 2009 and took some interesting photographs of the local based F-16s landing with crosswinds.





The following pictures are not crosswinds-related but show the 37° Stormo operations in a normal day of activity at Trapani – Birgi airbase.





FedEx MD11 crashes in Narita

On Mar. 23, a FedEx MD-11F with registration N526FE crashed during landing on the runway 34L/16R in Narita International, Tokyo, at 06.50 AM LT. Unfortunately both pilots died in the catastrophic crash. The aircraft was flying as FDX80 from Guangzhou – Bayiun, China, to Memphis, USA, via Narita and Anchorage, when around 21.50 Zulu (Sunday Mar. 22) the aircraft had a couple of bounces on landing prior to roll to the left and explode fuselage upside down. At the time of the crash, the airport was under strong winds: 222130Z 32026G40KT 9999 FEW020 12/M02 Q0999 WS R34L NOSIG RMK 1CU020 A2952 P/RR.
The video of the crash was immediately released:

The footage shows the aircraft bouncing hard on the runway after a flare (maybe with a high sink rate and not enough flare), ascending to something around 50 feet, then pitching down towards the tarmac, rolling slightly to the left. On the second touchdown the left Main Landing Gear collapses. As a consequence, the left wing goes into the ground and drags while the other wing raises into the air still producing lift. The reason for the pitch down attitude immediately after the first bounce is subject to speculations: someone thinks the pilot overcorrected to get the plane back down on the runway, others hypothesized the first bounce could have caused some problem with the flight control/surfaces.
This accident, reminded me of a previous FedEx MD-11 that crashed in Newark in 1997. Here’s the abstract of the NTSB report (available in pdf format here: http://www.ntsb.gov/Publictn/2000/AAR0002.pdf):

Abstract: On July 31, 1997, about 0132 eastern daylight time, a McDonnell Douglas MD-11, N611FE, operated by Federal Express, Inc., (FedEx) as flight 14, crashed while landing on runway 22R at Newark International Airport, Newark, New Jersey (EWR). The regularly scheduled cargo flight originated in Singapore on July 30 with intermediate stops in Penang, Malaysia; Taipei, Taiwan; and Anchorage, Alaska. The flight from Anchorage International Airport to EWR was conducted on an instrument flight rules flight plan and operated under the provisions of 14 Code of Federal Regulations Part 121. On board were the captain and first officer, who had taken over the flight in Anchorage for the final leg to EWR, one jumpseat passenger, and two cabin passengers. All five occupants received minor injuries in the crash and during subsequent egress through a cockpit window. The airplane was destroyed by impact and a postcrash fire.

The National Transportation Safety Board determines that the probable cause of this accident was the captain’s overcontrol of the airplane during the landing and his failure to execute a go-around from a destabilized flare. Contributing to the accident was the captain’s concern with touching down early to ensure adequate stopping distance.

Safety issues discussed in this report focus on landing techniques, bounced landing recovery, and training tools and policies that promote proactive decision-making to go around if an approach is unstabilized. Safety issues also include the use of on board computers to determine the required runway length for landing, MD-11 handling characteristics and structural integrity requirements, and hard landing inspection requirements. Tracking hazardous materials continues to be a safety issue and is also discussed in the report.

Look at the following youtube video showing another FedEx MD-11 (N587FE) landing on Feb 14 2009, on the same airport (Narita) with strong crosswinds.

I’ve already discussed about crosswinds landing risks and techniques when analysing the Lufthansa A320 LH044 wingstrike in Hamburg: http://cencio4.wordpress.com/2008/03/04/lufthansa-a320-wingstrike-at-hamburg/

Lufthansa A320 wingstrike at Hamburg

I’ve been receiving a lot of questions dealing with the LH Airbus 320 D-AIQP crosswind landing incident occurred on Mar. 1 at Hamburg airport. For those who don’t know yet what happened, I suggest having a look at the following amateur video that is also available on youtube and liveleak:

Flight LH 044 from Munich was approaching runway 23 under strong crosswinds when it was invested by a gust right before touchdown that raised the right wing leading to an unstable flare. The left wingtip slided along the runway before the engine provided enough thrust for a go around.

Fortunately, despite a lower half of winglet bent inboard and a slat partly damaged, the aircraft was able to perform another approach and land at the second attempt (at 13.55LT), even if there’s no video available of the A320 landing safely (it would have been interesting to see the different approach flown by the PIC after the “incident” with the left wing slightly damaged).

First of all, answering to those who wonder if the pilot is a hero or made a mistake, I think it the video is not sufficient to decide. Hower, the analysis of the available video (and pictures) can suggest a few hypothesis.

Let’s look at the weather conditions at Hamburg. The METAR for the airport at 12.20Z / 13.30LT reports:
EDDH 011220Z 29028G48KT 9000 -SHRA FEW011 BKN014 07/05 Q0984 TEMPO 29035G55KT 4000 SHRA BKN008

The actual wind conditions was 28G48KT (meaning 28 Knots with gusting up to 48 KTS) with a forecasted speed (TEMPO group, that indicates a temporary significant change within two hours from the observation) of 35 KTS with gusts to 55 KTS.

Since the aircraft approached RWY 23, considering the magnetic variation and the actual wind direction of 290°, the xwind (crosswind) component, according to my Jeppesen Model CR-3 Computer, was 25G42KT with forecasted xwind component of 31G48KT.

That said, considering that the demonstrated maximum xwind for the A320 is below 40KTS and that the actual conditions at the destination airport during approach are not known since the METAR is just an average calculation (in order to know the actual wind we should listen to the ATC comms, when the TWR controller read the windcheck to the LH044 before clearing it to land on RWY 23) there’s no evidence of a pilot (either Cdr or FO) mistake. A xwind of 25KTS would be still below the demostrated maximum xwind of the A320 (33KTS gusting 38KTS) even if the gusts would be well above it (although it must be remembered that the demostrated maximum xwind is not a limitation but just the maximum component experienced during aircraft testing).

Diverting to the alternate was an option but, as just said, weather condition could have been suitable for a safe landing considering also that the Emma storm on Central Europe most probably affected all the diverting airfields.

What is less clear is why the pilot did not decide to land on the other RWY available in HAM. In fact, using RWY 33, he would have faced a xwind component with angled by 40° instead of 60°. Using RWY 33, the 28G48KT wind would have meant a xwind component of “only” 18G32KTS with a forecated 24G36KTS. Assuming for example an actual wind of 40Kts from 290° during the approach, the xwind component would have been 35kts on RWY 23 and 26kts on RWY 33: a huge difference even for an experienced pilot.

Furthermore, RWY 33 is longer than 23 (3.666mt vs 3.250 mt) and is equipped with an ILS (Instrumental Landing System) and it’s a pilot responsibility to land on either runway, not an ATC one.

Dealing with the approach, it looked more or less stable until the very short final (not different from many others you can see by clicking here), around 60ft or so. According to the common procedure used with xwind components, the aircraft is correctly crabbed (meaning that it has applied a WCA, Wind Correction Angle, aligning nose and tail with the wind direction while the aircraft is following a different course). Just before touchdown, the pilot tries to reduce the WCA angle in order to prevent landing gear damage; however the “decrab” is extremely important on dry runways, because of the side load that could stress the gear on the gear, while it is not that important on wet runways like Hamburg one on Mar. 1 video.

It seems like the gust that moved the A320 outside the runway acted when the aircraft had been “decrabbed” and aligned with the runway. That is the phase of the landing with xwind in which the airplane is most vulnerable to the gustings and for this reasons, all aircraft (from the little C-152 to the MD-11 with some differences) have to apply cross-controls: left rudder, right aileron (if wind is coming from starboard) meaning rudder and aileron in opposite directions. This doesn’t mean that the aircraft has always to lower the wing on the upwind side (like I did with the Cessna), but this may be required to keep the aircraft on the runway even though many experienced pilots landing on dry runway are able to land with levelled wings.

So, one of the hypothesis, is that the pilot flying the aircraft did not apply cross-controls before touchdown or did apply the corrective input too late (or too slowly).

In order to understand the crabbing and decrabbing procedure, I suggest watching this interesting video:

The recovery was instead perfect. Since the engines need some seconds to spool up from Idle (from 3 to 5 I think) the crew must have started the go around as soon as the aircraft rolled dangerously to port, then the pilot maintained a low pitch and a smooth rate of climb in order to gain speed before trying another approach.