OPINION OF THE COURT
FORMAN, Circuit Judge.This is the second appeal to reach us in the wrongful death action brought by appellants in the United States District Court for the Eastern District of Pennsylvania, individually, as guardian and as executors of the estate of the deceased, Carlos Berguido, Jr. [hereinafter referred to as plaintiff, plaintiff-appellant, and appellant], against Eastern Air Lines [hereinafter referred to as defendant, defendant-appellee, and appellee]. Decedent was a passenger for hire on a constellation that crashed near Imeson Airport, Jacksonville, Florida in the early morning of December 21, 1955.
Following two mistrials, a third trial, before a jury, resulted in a verdict for plaintiff. The jury found that the crash was the result of one or more acts of wilful misconduct on the part of the defendant. Defendant’s attempt to limit its liability to $8300 under the Warsaw Convention1 governing international airplane flights was thus defeated.8 We reversed on evidentiary grounds.2 3
A fourth trial, before a jury, produced a verdict for plaintiff, but only for the limited Warsaw Convention liability of $8300, this jury resolving the issue of wilful misconduct adversely to the plaintiff. However, upon plaintiff’s motion, the District Court vacated its judgment and granted a new trial.4 Defendant applied to this court for a writ of mandamus and prohibition, arguing that the District Court had exceeded its power in granting the new trial. We denied the writ5 as well as a petition for reargument.6
The case was then scheduled for its fifth trial. The posture of the case was as it was at the previous trials. Defendant, having pleaded the Warsaw Convention, was attempting to limit its liability to $8300,7 while plaintiff’s main efforts were directed at proving either that the decedent’s flight was not international, or that defendant was guilty of wilful misconduct so as to otherwise eliminate the Warsaw Convention defense. The plaintiff had presented alternate theories of defendant’s wilful misconduct. It was urged by plaintiff that the pilot of the Eastern constellation had attempted a “sneak-in” landing *876—dropping down through a low cloud ceiling into a position below the authorized Instrument Landing System (ILS) approach minimum elevation. This theory was developed by plaintiff through the presentation of hypothetical questions to experts, Glickstein, Kepner and Cann. Plaintiff’s alternative theory was that both the pilot and the co-pilot operating the aircraft had submarginal heart conditions. Medical records were introduced in support of this theory.
Plaintiff alleges that it also urged that defendant had not met its obligations under Article 3(1) of the Convention8 and, thus, the Convention did not apply. Plaintiff appears to indicate that the District Court was thus urged in the alternative to try the case on the theory of simple negligence.
The parties waived trial by jury. They also agreed that the District Court need not enter findings of fact and conclusions of law, which are required, by Federal Rule of Civil Procedure 52(a) in non-jury cases. The parties further specified that the District Court might limit its decision to a general finding either for or against liability and in a single total sum fixing damages, just as a jury would do. Despite the acquiescence by the parties to a short form of disposition, not authorized by the Federal Rules, the District Court did file an opinion9 in which it set forth findings of fact on certain aspects of its ruling.
Summarily determining the international nature of the flight, a finding which the plaintiff does not now contest, the bulk of the District Court’s opinion is devoted to a discussion of whether the defendant was guilty of wilful misconduct. A discussion of whether defendant complied with Article 3(1) of the Warsaw Convention is absent, although the Convention was determined to be applicable. The District Court observed that this was a case where the evidence of wilful misconduct was meager and incomplete, making the search for truth a most difficult task. It stated that a considerable number and variety of causes of the accident were within the realm of possibility. The District Court observed further that in a case such as this, where the search is for the cause of the accident, the judgment of a court must be determined upon the probabilities insofar as they can be determined from the evidence, and not on a mere possibility, even if such possibility be disclosed by the evidence.
The above preliminary observations were followed by a statement of “firm findings” which the District Court apparently considered facts found “spe-*877dally” conforming to the requirement of Federal Rule of Civil Procedure 52(a). The District Court found that there was not a preponderance of credible evidence that a health condition constituted wilful misconduct causing or contributing to the crash of the constellation. As to the “sneak-in” theory, the District Court stated that it could not find as proven by the preponderance of the evidence all of the assumed facts stated in the hypothetical questions put to plaintiff’s experts. But, even if all the assumed facts were taken to be proven, said the District Court, the conclusions reached by plaintiff’s experts were not believable. Rather, the District Court chose to find that there was a substantial and reasonable probability that the presence of jet aircraft in or near the ILS glide slope for the Eastern constellation interfered with the aircraft, bringing about emergency conditions resulting in the accident, the theory of the defendant. The Court then concluded that defendant was guilty neither of wilful misconduct nor of negligence. Having previously found the Warsaw Convention applicable, judgment Wc.s entered against the defendant for $8300. Though finding against defendant’s liability above $8300, the District Court indicated, albeit quite unnecessarily, that plaintiff had suffered $375,000 in damages.
—I—
Prior to examining plaintiff-appellant’s position on this appeal, a question of some moment is whether the District Court complied with Federal Rule of Civil Procedure 52(a). The agreement between the parties, that the District Court need not enter findings of fact, is not effective. The requirement of Rule 52(a) is mandatory, the findings in part serving as a necessary aid to the appellate courts.10 If a full understanding of the factual issues cannot be gleaned from the District Court’s opinion, we would be obliged to remand for compliance with Rule 52(a).
Were the findings, as prepared, sufficient compliance with Rule 52(a)? Concerning plaintiff’s theory of the submarginal heart conditions of the pilot and co-pilot, the single finding that there was no probable causal relationship shown between their heart conditions and the accident is ample to meet the requirement of Rule 52(a). The basis of the Court’s ruling is clear and an examination of the record will reveal whether there is substantial support, for it.
The District Court did not, however, comply with Rule 52(a) when it found, without further elaboration, that some of the assumed facts stated in the hypotheticals were not proven. But for the Court’s further assuming the validity of all the facts in the hypotheticals for purposes of testing whether the defendant, even under those circumstances, was guilty of wilful misconduct, the failure to elaborate on the inadequacy of the hypotheticals would have been a fatal defect requiring a remand. The Court did find, however, that assuming the accuracy of the facts in the hypotheticals, plaintiff’s expert witnesses, Glickstein, Kepner and Cann, drew a conclusion therefrom as to the probable cause of the accident that was not believable. Considerable effort was exercised by the Court in analyzing the basis for the refusal to accept their conclusion. Such elaboration qualifies the finding as sufficient under Rule 52(a), if indeed a specific finding by the trier of fact is needed when the issue is one of credibility.
As to the District Court’s finding that jet aircraft in or near the ILS glide slope interfered with the Eastern constellation and, therefore, defendant was not guilty of wilful misconduct, this states a fact susceptible of examination by an appellate court and thus is in compliance with Rule 52(a).
As an initial matter we conclude, therefore, that the requirements of Rule *87852(a) have been sufficiently met.11 The findings of fact and conclusions of law may now be considered in the light of plaintiff-appellant’s appeal contentions.
—II—
Appellant now concedes the international nature of the flight and to this extent the applicability of the Warsaw Convention. It is contended for the first time, however, that the litigants to this suit, as well as most courts which have previously dealt with the question of liability under the Warsaw Convention, have labored under a misconception concerning the scheme of the Convention. Appellant urges a theory of unlimited absolute liability in the face of the ap-pellee’s alleged failure to comply with Article 3(1) of the Convention. It is argued that Article 1712 provides for unlimited absolute liability unless a carrier proves that it has complied with Article 3(1). Appellee, urges appellant, having failed to carry its burden of proof is, as a matter of law, liable without fault for the total amount of appellant’s damages, under Article 17 of the Convention. Specifically, appellant contends that ap-pellee has failed to prove both actual delivery of the ticket to the decedent and actual delivery of an adequate ticket showing (1) all agreed stopping places and (2) the provisions of the Convention which limit a carrier’s liability. Although appellant’s contention is that the burden of proof is on the carrier, an attempt is also made by appellant to affirmatively show the inadequacy of the ticket, although not the failure to actually deliver that ticket. However, failing acceptance of this approach, appellant urges that the District Court’s finding of no wilful misconduct by the appellee was clearly erroneous.
Appellee contends that the issue of compliance with Article 3(1) of the Convention was eliminated from the ease at pre-trial and that, nevertheless, a plaintiff, and not a carrier-defendant, has the burden of proving non-compliance with Article 3(1). Actual non-delivery of the ticket under Article 3(2) is also urged as the only Article 3(1) conduct of a carrier which will defeat the Convention as a defense. As to the question of wilful misconduct, appellee urges that there was substantial basis for the District Court’s exonerating it therefrom. Appellee furthermore cross-assigns as error the District Court’s grant of the motion for a new trial subsequent to the jury verdict in its favor at the fourth trial, which resulted in the fifth trial from which this appeal has been taken.
The contentions thus having been outlined, we now turn to their disposition.
-III-
CS] Appellant has urged that both Grey v. American Airlines 13 and Westminster Bank v. Imp. Airways, Ltd.14 stand for the proposition that liability under Article 17 of the Warsaw Convention, as an initial matter, is absolute and unlimited. To avoid such liability a carrier must demonstrate compliance with Article 3(1) of the Convention, argues appellant. Neither case supports the proposition, for the question of whether Article 17 provides such unlimited absolute liability for the death of a passenger was not an issue therein. Furthermore, in the first appeal before this court, *879Judge McLaughlin, though not faced squarely with the issue as here raised, in effect disposed of it by appropriately reading Articles 17 and 22(1)15 in pari materia. It was thus stated:
“ * * * The Warsaw Convention provides, inter alia, that the carrier is absolutely liable for all injuries where the accident causing the damage so sustained took place on board the aircraft. Article 17. In such circumstances the liability of the carrier for each passenger is limited to 125,000 francs (approximately $8300). Article 22(1). * * * » 16
Article 17 itself gives no indication of the extent of a carrier’s liability. It must be read with Article 22(1) for the question concerning the extent of the liability to be answered, and an analysis of the question does not support absolute unlimited liability as the scheme of the Convention.
Having thus disposed of this contention, appellant must sustain either (1) its argument that the District Court erroneously found that the appellee was free from wilful misconduct or (2) its argument that there had not been compliance with Article 3(1) of the Convention. Success of the latter argument would have removed the Convention defense of limited liability and allowed the suit to proceed to trial, with the plaintiff’s burden being merely to prove the negligence of the defendant, rather than wilful misconduct, this approach being the appropriate substitute for appellant’s urged absolute and unlimited liability without fault theory.
Such contention, revolving around a construction of Article 3(1), raises the more specific issues of (a) which party has the burden of proof to show compli-anee or a failure thereof, (b) whether the issue as framed by appellant was eliminated at pre-trial, (c) whether the record before us is sufficient to allow us to dispose of the merits of the mixed questions, and (d) if the record is ample, whether appellee has complied with Article 3(1) and what are the legal ramifications if it has not done so.17
Appellant submits that its position relative to Article 3(1) must be sustained, that it proved negligence, and that, therefore, this court should remand for entry of judgment in the amount of $375,000 as found by the District Court. The District Court, however, as well as finding that appellee was not guilty of wilful misconduct, found that appellee was not even negligent. If this conclusion is supportable, it is final and there is no need for us to examine appellant’s legal contentions concerning Article 3(1) of the Convention, which we have set forth in the foregoing paragraph, for even if those contentions are determined in favor of appellant, they would not dictate a reversal.
The issues of appellee’s wilful misconduct and negligence are bound together in this ease, for appellant’s theories of the cause of the accident would have been the basis for proving both, and the burden of such proof as to both remained with appellant as plaintiff.18
As to appellee’s theory that jets entered the ILS glide slope of the Eastern constellation, and the District Court’s acceptance of it as the probable cause of the accident, it appears that the evidence supporting it is exceedingly thin. It is unnecessary in this instance to finally resolve whether such a finding is supported by substantial evidence in the record for, even absent such a find*880ing, we are satisfied that the record supports the District Court’s conclusion that appellant failed to prove conduct of the appellee that was either wilful or negligent, and the proximate cause of the accident. Such independent proof would have been essential to appellant’s recovery.
Though medical deficiencies of the pilot and co-pilot were demonstrated, the District Court chose not to draw an inference from this fact that a triggering of known physical disabilities was the cause of the air mishap. It had the right as the trier of fact to refuse to draw such an inference. As there was no other evidence introduced to show either a physical malfunction of the pilot or co-pilot or that such a malfunction was the probable cause of the accident, the District Court’s finding that this theory could not be sustained as the probable cause of the accident is not clearly erroneous.
Appellant’s alternative theory that the pilot knowingly attempted an illegal “sneak-in” below the cloud ceiling in violation of minimal altitude requirements rested solely on the answers to hypothetical questions put to three expert witnesses by appellant. Appellant’s experts’ answers to the hypothetical all concluded that the pilot must have been attempting the theorized illegal “sneak-in”. Appellee's experts testified that it was not possible to draw such a conclusion from the hypothetical facts. The District Court chose not to believe appellant’s experts. Its impression of appellant’s experts as stating overly partisan, highly speculative, and unqualified opinions was a permissible determination of credibility by the fact finder. Thus, as the testimony of the expert witnesses was the only basis for appellant’s “sneak-in” theory, the District Court’s refusal to believe it, particularly in the light of contrary expertise, was well within its province as fact finder.
The initial observation of the District Court that the evidence as to causation was meager and incomplete would appear to have substantial support in the record. Its conclusion that the evidence was not sufficient for the appellant, as plaintiff, to have sustained the requisite burden of proof on the question of ap-pellee’s wilful misconduct or negligence was not clearly erroneous. The resolution of this issue which is fundamental to the litigation, against appellant, precludes the necessity for our determining the numerous subsidiary issues it has raised, as noted heretofore.19
The District Court entered judgment in favor of appellant, but for the limited Convention liability of $8300. Appellee did not contest this entry of judgment. Indeed, it tendered the $8300 prior to judgment and has taken no cross-appeal. The judgment of the United States District Court for the Eastern District of Pennsylvania of September 30, 1964 will, therefore, be affirmed.
. Article 22(1) of the Convention for the Unification of Certain Rules Relating to International Transportation by Air (Warsaw Convention), 49 Stat. 3014, 3019 (1936) states:
“In the transportation of passengers the liability of the carrier for each passenger shall be limited to the sum of 125,000 francs. * * * ”
. Article 25(1) of the Warsaw Convention, 49 Stat. 3014, 3020 (1936) provides:
“The carrier shall not be entitled to avail himself of the provisions of this convention which exclude or limit his liability, if the damage is caused by his wilful misconduct or by such default on his part as, in accordance with the law of the court to which the case is submitted, is considered to be equivalent to wilful misconduct.”
. Berguido v. Eastern Air Lines, 317 F.2d 628, rehearing denied, 317 F.2d 633 (3 Cir.), cert. denied, 375 U.S. 895, 84 S.Ct. 170, 11 L.Ed.2d 124 (1963).
. Berguido v. Eastern Air Lines, Inc., 35 F.R.D. 200 (E.D.Pa.1964).
. Eastern Air Lines Inc. v. Clary, No. 14940, 3 Cir., June 18, 1964.
. Eastern Air Lines, Inc. v. Clary, No. 14940, 3 Cir., July 10, 1964.
. Defendant conceded that it could not sustain its burden of proof under Article 20 (1) of the Warsaw Convention which states:
“The carrier shall not be liable if he proves that he and his agents have taken age or that it was impossible for him or all necessary measures to avoid the dam-them to take such measures.” 49 Stat. 3014, 3019 (1936).
. Plaintiff contends that Article 3(1) of the Convention, 49 Stat. 3014, 3015 (1936) enumerates the conditions precedent to the applicability of the Convention and that the failure of the defendant carrier to so comply both removes the Convention as a defense and places the burden on plaintiff merely of proving negligence. Article 3(1) provides:
“(1) For the transportation of passengers the carrier must deliver a passenger ticket which shall contain the following particulars:
“(a) The place and date of issue;
“(b) The place of departure and of destination;
“ (c) The agreed stopping places, provided that the carrier may reserve the right to alter the stopping places in case of necessity, and that if he exercises that right, the alteration shall not have the effect of depriving the transportation of its international character;
“(d) The name and address of the carrier or carriers;
“(e) A statement that the transportation is subject to the rules relating to liability established by this convention.” Article 3(2) provides further, however,
that:
“The absence, irregularity, or loss of the passenger, ticket shall not affect the existence of the validity of the contract of transportation, which shall none the less be subject to the rules of this convention. Nevertheless, if the carrier accepts a passenger without a passenger ticket having been delivered he shall not be entitled to avail himself of those provisions of this convention which exclude or limit his liability.”
. Berguido v. Eastern Air Lines, Inc., 35 F.R.D. 200, E.D.Pa., Sept. 30, 1964.
. 5 Moore, Federal Practice § 52.06[1], at 2653 (2d ed. 1951).
. The failure of the District Court to discuss the issue of defendant’s complying with Article 3(1) of the Warsaw Convention, supra note 8, seems to be due to its understanding of the pre-trial limitation of the issues. This problem will be discussed and disposed of-below in a different context, however, and need not further clutter the issue of the Court’s compliance with Rule 52(a).
. Article 17, 49 Stat. 3014. 3018 (1936) reads:
“The carrier shall be liable for damage sustained in the event of the death or wounding of a passenger or any other bodily injury suffered by a passenger, if the accident which caused the damage so sustained took place on board the aircraft or in the course of any of the operations of embarking or disembarking.”
. 227 F.2d 282 (2 Cir. 1955).
. 2 All E.R. 890 (K.B.1936).
. Supra note 1.
. Berguido v. Eastern Air Lines, supra note 3 at 629 of 317 F.2d.
. For discussions of the legal consequences of a failure to comply see Mertens v. Flying Tiger Line, Inc., 341 F.2d 851 (2 Cir. 1965); Grey v. American Airlines, supra note 13 at 284 affirming 95 F.Supp. 756 (S.D.N.Y.1950) ; Lisi v. Alitalia-Linee Aeree Italiane, 253 F.Supp. 237 (S.D.N.Y.1966).
It is in passing noteworthy that in Mertens the plaintiff had assumed that the burden was upon him to prove defendant’s failure to comply with Article 3(1).
. Grey v. American Airlines, supra note 13 at 285-286.
. This disposition also makes it unnecessary to consider appellee’s cross-assignment of error concerning the District Court’s grant of appellant’s new trial motion subsequent to the fourth trial and the jury verdict in appellee’s favor.