L'Hotellier Connectors

typical l'hotellier connector (this is a 90 degree one)The DG-200 uses l'hotellier connectors for the aileron, airbrake and elevator linkages. The first two of these (4 connectors) are accessed through a small hatch a short distance behind the canopy on top of the fuselage. The elevator connection was automated with the launch of the DG-202 and this removed a major safety hazard. Whilst fairly standard for the period (many much later aircraft use them), l'hotellier connectors have a relatively poor safety record and there have been instances of ailerons and airbrakes and even elevators disconnecting in flight or being incorrectly coupled during rigging. On the DG-200 the ailerons are fairly easy to locate and connect at the rear of the hatch but the airbrakes situated closer to the spars can be more difficult. The closed position of the brakes may not coincide with the 'closed but not locked' position of the control lever and therefore require some force to engage. It is only possible to get one hand to the connectors and it is not easy (or impossible!) to see them at the same time. It is worthwhile adjusting the sockets on the airbrake connectors so that they approach the balls at a convenient angle to make it easier to apply the required pressure whilst holding the locking cams in the unlocked position.

l'hotellier socket diagram showing cam plate and inspection hole for safety pin
Diagram of the socket of a l'hotellier showing the cam plate and inspection hole

To connect the l'hotellier, the cam plate is depressed from its open position so that the narrower side is flush with the body of the cup. The cup is slipped over the ball on the other part of the coupling and the cam plate is released. If the ball is correctly located, the cam plate will spring back to the 'closed and locked' position in which the inspection hole will be completely visible as shown above.

l'hotellier socket diagram with inspection hole for safety pin only partly visible
Diagram of the socket of an incorrectly coupled l'hotellier showing the cam plate with inspection hole not completely visible.

If the ball is not correctly located then either the cam plate will spring back to the open position or else the inspection hole will not be visible. The coupling should then be pulled and twisted sharply to make sure that it is properly engaged. It should not be possible to disengage the coupling even with the application of very considerable force or a sharp jerk.

These connectors have been the subject of airworthiness directives - the LBA directives are 1993-001/3 and 1994-001/2 - the former is relevant to the DG-200. The LBA have found that it is possible for l'hotelliers, even when in good condition and correctly connected, to come open under the normal stresses of flight. A minimum mandatory safety requirement is that the locking cams are secured with an appropriate pin. This is fitted in the hole originally intended for visual verification of locking and the hole may need to be bored out to take a suitable pin. Note that the pins need to be a good fit otherwise the whole point is lost. It has been demonstrated for instance that using an ordinary safety-pin instead of the correct pin can allow even an only-slightly-worn connector to disengage. Unfortunately fitting these pins is not easy especially with cold hands and unless the pins are secured with a handy piece of string it is all too easy to drop them to the floor of the fuselage. Again it is worthwhile adjusting the sockets so that the locking cams are easy to get to. Clearly it is possible to insert a pin into an unlocked coupling (to no advantage)! Also a worn coupling may allow a pin to be inserted in the 'closed but not locked' position above, again to no advantage.

A better (and recommended) solution is the use of some form of locking mechanism such as the Uerlings sleeve, LS sleeve, Schempp-Hirth spring and Wedekind locking system - the last being possibly the most reliable and easiest to use. It's not very likely that you'll find the LS or Schempp-Hirth safeties on a DG-200 l'hotellier. The Uerlings sleeve though approved is known to be prone to distorting and cracking and is no longer recommended by many inspectors. The manufacturers themselves required their replacement every two years.

L'Hotellier Safety Mechanisms Found On DG-200 / DG-202
l'hotellier with end view showing safety pin fitted The minimum safety requirement as shown here is the fitting of a safety pin through the inspection hole of the operating cam. The inspection hole is (re-)sized so that when the cam plate is fully back with the connector properly made the hole just clears the shell. The safety pin can thus only be fitted if the connector is fully made and the cam fully withdrawn and prevents the cam from being pushed or shaken in by severe shocks such as might be obtained during a bumpy takeoff. Provided the ball and socket are within tolerance they cannot separate while the cam plate is in this position. It's important to verify that the pin locks the cam plate firmly so that the ball has no opportunity to be jarred loose. It's therefore important that the pin is correctly sized for the hole.
l'hotellier side view with Uerling sleeve The Uerling sleeve is a slotted plastic ring on the socket side of the connector which is rotated to cover the ball joint after the connection has been made as shown. The idea is that there is relatively little force available to dislocate the ball from its socket (normally this would be not much more than the inertia of the control linkages) and so relatively little is required to retain it. It is not unknown for the sleeve to become cracked or distorted so that the locking action is reduced or lost. It can only be used with straight couplings.
l'hotellier side view with Wedekind locking system The Wedekind locking system consists of a spring-loaded sleeve on the socket side of the connector as shown. The sleeve is retracted to allow the connection to be made as normal and when released slides forward to prevent the cam plate from moving. The sleeve will not engage unless the connection has been properly made and the cam plate is fully home. This can easily be accomplished in one single-handed manoevre. It can be used with both straight and 90o couplings. (Webmaster's personal choice - see Glaser-Dirks TN323/7)

Whatever safety means are employed it is important to be particularly careful of these couplings when rigging. Suggestions from other sources include:

"It is true that the roll effectiveness is higher when negative flaps are set, expecially if the ailerons move together with the flaps as in the DG-200. But one day last year, a friend of mine departed in an ASH-25 with the center (flaperon) aileron on the left wing not connected. He completed an aerotow circuit and managed to land safely. His passenger (an inexperienced pilot) reported that during the flight (and, after the flight, on the ground) the unconnected control surface drooped to its full down position and possibly because of its own weight was not being sucked upwards by the airflow. This caused the plane to roll right, which the pilot needed to correct by applying full left aileron. On final approach, he selected a positive flap setting, which caused all other control surfaces to come down too, and now the pilot had some roll control again. Setting a negative flap in this case would probably have aggavated the imbalance and caused the ASH-25 to roll right, out of control. So I think one should never immediately set one or the other flap setting in such a situation, but have a few seconds to judge the situation. Perhaps in another flapped glider, the aileron would stick in the up position or be sucked more or less neutral, one can not tell that in advance I think. And you must look to see which aileron has disconnected and realise whether the right wing is going up or the left wing is going down.

On lower-chord wing profiles such as the Libelle, an unconnected aileron is reported to initiate a low-frequency oscillation which the pilot has to correct all the time with the remaining aileron."

Thanks for all of these suggestions.

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